Data transmission method in wireless communication system and device therefor

ABSTRACT

An AP downlink multi-user transmission method in a wireless communication system, according to one embodiment of the present invention, comprises the steps of: generating a DL MU PPDU, wherein the DL MU PPDU comprises resource allocation information for uplink MU transmission by a station (STA); transmitting the DL MU PPDU to the STA; and, on the basis of the DL MU PPDU, receiving a UL MU PPDU generated by the STA, wherein the UL MU PPDU comprises a first part having a first IDFT/DFT cycle, and a second part having a second IDFT/DFT cycle that is four times the first IDFT/DFT cycle, wherein the first part may be received through at least one 20 MHz channel in a position corresponding to a frequency resource indicated by the resource allocation information, and the second part may be received by using the frequency resource indicated by the resource allocation information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2015/013771, filed on Dec. 15, 2015,which claims the benefit of U.S. Provisional Application No. 62/092,266,filed on Dec. 16, 2014, the contents of which are all herebyincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to wireless communication systems, andmore particularly, to a method for transmitting data for supporting adata transmission of multi-user and a device for supporting the same.

BACKGROUND ART

Wi-Fi is a wireless local area network (WLAN) technology which enables adevice to access the Internet in a frequency band of 2.4 GHz, 5 GHz or60 GHz.

A WLAN is based on the institute of electrical and electronic engineers(IEEE) 802.11 standard. The wireless next generation standing committee(WNG SC) of IEEE 802.11 is an ad-hoc committee which is worried aboutthe next-generation wireless local area network (WLAN) in the medium tolonger term.

IEEE 802.11n has an object of increasing the speed and reliability of anetwork and extending the coverage of a wireless network. Morespecifically, IEEE 802.11n supports a high throughput (HT) providing amaximum data rate of 600 Mbps. Furthermore, in order to minimize atransfer error and to optimize a data rate, IEEE 802.11n is based on amultiple inputs and multiple outputs (MIMO) technology in which multipleantennas are used at both ends of a transmission unit and a receptionunit.

As the spread of a WLAN is activated and applications using the WLAN arediversified, in the next-generation WLAN system supporting a very highthroughput (VHT), IEEE 802.11ac has been newly enacted as the nextversion of an IEEE 802.11n WLAN system. IEEE 802.11ac supports a datarate of 1 Gbps or more through 80 MHz bandwidth transmission and/orhigher bandwidth transmission (e.g., 160 MHz), and chiefly operates in a5 GHz band.

Recently, a need for a new WLAN system for supporting a higherthroughput than a data rate supported by IEEE 802.11ac comes to thefore.

The scope of IEEE 802.11ax chiefly discussed in the next-generation WLANtask group called a so-called IEEE 802.11ax or high efficiency WLAN(HEW) includes 1) the improvement of an 802.11 physical (PHY) layer andmedium access control (MAC) layer in bands of 2.4 GHz, 5 GHz, etc., 2)the improvement of spectrum efficiency and area throughput, 3) theimprovement of performance in actual indoor and outdoor environments,such as an environment in which an interference source is present, adense heterogeneous network environment, and an environment in which ahigh user load is present and so on.

A scenario chiefly taken into consideration in IEEE 802.11ax is a denseenvironment in which many access points (APs) and many stations (STAs)are present. In IEEE 802.11ax, the improvement of spectrum efficiencyand area throughput is discussed in such a situation. More specifically,there is an interest in the improvement of substantial performance inoutdoor environments not greatly taken into consideration in existingWLANs in addition to indoor environments.

In IEEE 802.11ax, there is a great interest in scenarios, such aswireless offices, smart homes, stadiums, hotspots, andbuildings/apartments. The improvement of system performance in a denseenvironment in which many APs and many STAs are present is discussedbased on the corresponding scenarios.

In the future, it is expected in IEEE 802.11ax that the improvement ofsystem performance in an overlapping basic service set (OBSS)environment, the improvement of an outdoor environment, cellularoffloading, and so on rather than single link performance improvement ina single basic service set (BSS) will be actively discussed. Thedirectivity of such IEEE 802.11ax means that the next-generation WLANwill have a technical scope gradually similar to that of mobilecommunication. Recently, when considering a situation in which mobilecommunication and a WLAN technology are discussed together in smallcells and direct-to-direct (D2D) communication coverage, it is expectedthat the technological and business convergence of the next-generationWLAN based on IEEE 802.11ax and mobile communication will be furtheractivated.

DISCLOSURE Technical Problem

An object of the present invention is to propose an uplink/downlinkmulti-user data transmission and reception method in a wirelesscommunication system.

Furthermore, an object of the present invention is to propose a highefficiency (HE) format of a PPDU used in uplink/downlink multi-usertransmission/reception in a wireless communication system. Morespecifically, the present invention proposes an HE format of an UL MUPPDU.

The technical objects of the present invention are not limited to thoseobjects described above; other technical objects not mentioned above maybe clearly understood from what are described below by those skilled inthe art to which the present invention belongs.

Technical Solution

A downlink (DL) multi-user (MU) transmission method of an access point(AP) in a wireless communication system according to an embodiment ofthe present invention includes generating a DL MU physical protocol dataunit (PPDU), the DL MU PPDU comprising resource allocation informationfor an uplink (UL) MU transmission of a station (STA), sending the DL MUPPDU to the STA, and receiving an UL MU PPDU generated by the STA basedon the DL MU PPDU. The UL MU PPDU may include a first part having afirst inverse discrete Fourier transform (IDFT)/discrete Fouriertransform (DFT) period and a second part having a second IDFT/DFT periodwhich is four times the first IDFT/DFT period, the first part may bereceived through at least one 20 MHz channel of a location correspondingto a frequency resource indicated by the resource allocationinformation, and the second part may be received using the frequencyresource indicated by the resource allocation information.

Furthermore, if the 20 MHz channel of the location corresponding to thefrequency resource is plural, the first part may be duplicated in 20 MHzunit and received through the plurality of 20 MHz channels.

Furthermore, the first part may be duplicated in 20 MHz unit andreceived through a full transmission channel of the DL MU PPDU.

Furthermore, if the at least one 20 MHz channel of the locationcorresponding to the frequency resource does not correspond to a primarychannel, the first part may be duplicated in 20 MHz unit and receivedthrough the at least one 20 MHz channel and the primary channel.

Furthermore, if a different 20 MHz channel is present between the atleast one 20 MHz channel and the primary channel, the first part may beduplicated in 20 MHz unit and received through the at least one 20 MHzchannel, the different 20 MHz channel, and the primary channel.

Furthermore, the first part may include a legacy (L)-short trainingfield (STF), an L-long training field (LTF), an L-signal (SIG) field,and a high efficiency (HE) SIG-A field, and the second part may includean HE-STF, an HE-LTF, and a data field.

Furthermore, an uplink (UL) multi-user (MU) transmission method of anSTA in a wireless communication system according to another embodimentof the present invention includes receiving a DL MU physical protocoldata unit (PPDU), the DL MU PPDU comprising resource allocationinformation for an UL MU transmission of a station (STA), and sending anUL MU PPDU generated based on the DL MU PPDU. The UL MU PPDU may includea first part having a first inverse discrete Fourier transform(IDFT)/discrete Fourier transform (DFT) period and a second part havinga second IDFT/DFT period which is four times the first IDFT/DFT period,the first part may be transmitted through at least one 20 MHz channel ofa location corresponding to a frequency resource indicated by theresource allocation information, and the second part may be transmittedusing the frequency resource indicated by the resource allocationinformation.

Furthermore, if the 20 MHz channel of the location corresponding to thefrequency resource is plural, the first part may be duplicated in 20 MHzunit and transmitted through the plurality of 20 MHz channels.

Furthermore, the first part may be duplicated in 20 MHz unit andtransmitted through a full transmission channel of the DL MU PPDU.

Furthermore, if the at least one 20 MHz channel of the locationcorresponding to the frequency resource does not correspond to a primarychannel, the first part may be duplicated in 20 MHz unit and transmittedthrough the at least one 20 MHz channel and the primary channel.

Furthermore, if a different 20 MHz channel is present between the atleast one 20 MHz channel and the primary channel, the first part may beduplicated in 20 MHz unit and transmitted through the at least one 20MHz channel, the different 20 MHz channel, and the primary channel.

Furthermore, the first part may include a legacy (L)-short trainingfield (STF), an L-long training field (LTF), an L-signal (SIG) field,and a high efficiency (HE) SIG-A field, and the second part may includean HE-STF, an HE-LTF, and a data field.

Furthermore, a station (STA) device of a wireless LAN (WLAN) systemaccording to another embodiment of the present invention includes an RFunit sending or receiving a radio signal and a processor controlling theRF unit. The processor may receive a DL MU physical protocol data unit(PPDU), the DL MU PPDU including resource allocation information for theuplink (UL) multi-user (MU) transmission of the STA, and sending an ULMU PPDU generated based on the DL MU PPDU. The UL MU PPDU may include afirst part having a first inverse discrete Fourier transform(IDFT)/discrete Fourier transform (DFT) period and a second part havinga second IDFT/DFT period which is four times the first IDFT/DFT period.The first part may be received through at least one 20 MHz channel of alocation corresponding to a frequency resource indicated by the resourceallocation information. The second part may be received using thefrequency resource indicated by the resource allocation information.

Furthermore, if the 20 MHz channel of the location corresponding to thefrequency resource is plural, the first part may be duplicated in 20 MHzunit and transmitted through the plurality of 20 MHz channels.

Furthermore, the first part may be duplicated in 20 MHz unit andtransmitted through a full transmission channel of the DL MU PPDU.

Furthermore, if the at least one 20 MHz channel of the locationcorresponding to the frequency resource does not correspond to a primarychannel, the first part may be duplicated in 20 MHz unit and transmittedthrough the at least one 20 MHz channel and the primary channel.

Furthermore, if a different 20 MHz channel is present between the atleast one 20 MHz channel and the primary channel, the first part may beduplicated in 20 MHz unit and transmitted through the at least one 20MHz channel, the different 20 MHz channel, and the primary channel.

Furthermore, the first part may include a legacy (L)-short trainingfield (STF), an L-long training field (LTF), an L-signal (SIG) field,and a high efficiency (HE) SIG-A field, and the second part may includean HE-STF, an HE-LTF, and a data field.

Advantageous Effects

In accordance with an embodiment of the present invention, a powerimbalance problem is not generated in each band (e.g., per 20 MHzchannel) because the first part (i.e., a portion to which the 64 FFTsize has been applied) is transmitted over a full band. Furthermore, anempty band (e.g., an empty 20 MHz channel) is not present because thefirst part is transmitted over a full band. Accordingly, there is anadvantage in that a full band (all of transmission channels) can besubjected to TXOP protection in the case of TXOP protection using anL-SIG or HE-SIG A field.

Furthermore, in accordance with another embodiment of the presentinvention, a collision attributable to the data transmission of anotherSTA can be prevented because the first part is transmitted through aprimary channel.

Furthermore, another embodiment of the present invention proposes an ULMU PPDU having small overhead and a more simplified structure.

In addition, other effects of the present invention are additionallydescribed in the following embodiments.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with the description, serve to explain the technical featuresof the invention.

FIG. 1 is a diagram showing an example of an IEEE 802.11 system to whichthe present invention may be applied;

FIG. 2 illustrates non-HT format PPDUs and HT format PPDUs in a wirelesscommunication system to which an embodiment of the present invention maybe applied;

FIG. 3 illustrates VHT format PPDU formats of a wireless communicationsystem to which an embodiment of the present invention may be applied;

FIG. 4 is a diagram illustrating a constellation for classifying thePPDU formats of a wireless communication system to which an embodimentof the present invention may be applied;

FIG. 5 illustrates a MAC frame format in an IEEE 802.11 system to whichthe present invention may be applied;

FIG. 6 is a diagram illustrating the frame control field in the MACframe in a wireless communication system to which the present inventionmay be applied;

FIG. 7 illustrates the VHT format of an HT control field in a wirelesscommunication system to which the present invention may be applied;

FIG. 8 is a diagram illustrating a downlink multi-user PPDU format in awireless communication system to which the present invention may beapplied;

FIG. 9 is a diagram illustrating a downlink multi-user PPDU format in awireless communication system to which the present invention may beapplied;

FIG. 10 is a diagram illustrating a downlink MU-MIMO transmissionprocess in a wireless communication system to which the presentinvention may be applied;

FIG. 11 is a diagram illustrating an ACK frame in a wirelesscommunication system to which the present invention may be applied;

FIG. 12 is a diagram illustrating a Block Ack Request frame in awireless communication system to which the present invention may beapplied;

FIG. 13 is a diagram illustrating the BAR Information field of a BlockAck Request frame in a wireless communication system to which thepresent invention may be applied;

FIG. 14 is a diagram illustrating a Block Ack frame in a wirelesscommunication system to which the present invention may be applied;

FIG. 15 is a diagram illustrating the BA Information field of a BlockAck frame in a wireless communication system to which the presentinvention may be applied;

FIG. 16 is a diagram illustrating a high efficiency (HE) format PPDUaccording to an embodiment of the present invention;

FIGS. 17 to 19 are diagrams illustrating a HE format PPDU according toan embodiment of the present invention.

FIG. 20 is a diagram illustrating an uplink multi-user transmissionprocedure according to an embodiment of the present invention.

FIG. 21 illustrates UL MU transmission according to an embodiment of thepresent invention;

FIG. 22 is a diagram illustrating a CTS-to-self frame according to anembodiment of the present invention;

FIG. 23 shows the structure of the UL MU PPDU of an HE format accordingto an embodiment of the present invention;

FIG. 24 is a diagram showing the structure of an UL MU PPDU according toa first embodiment of the present invention;

FIG. 25 is a diagram showing the structure of an UL MU PPDU according toa second embodiment of the present invention;

FIG. 26 is a diagram showing the structure of an UL MU PPDU according toa third embodiment of the present invention;

FIG. 27 is a flowchart regarding the DL MU transmission method of an APdevice according to an embodiment of the present invention; and

FIG. 28 is a block diagram of an STA device according to an embodimentof the present invention.

BEST MODE FOR INVENTION

Hereinafter, some embodiments of the present invention are described indetail with reference to the accompanying drawings. The detaileddescription to be disclosed herein along with the accompanying drawingsis provided to describe exemplary embodiments of the present inventionand is not intended to describe a sole embodiment in which the presentinvention may be implemented. The following detailed descriptionincludes detailed contents in order to provide complete understanding ofthe present invention. However, those skilled in the art will appreciatethat the present invention may be implemented even without such detailedcontents.

In some cases, in order to avoid making the concept of the presentinvention vague, the known structure and/or device may be omitted or maybe illustrated in the form of a block diagram based on the core functionof each structure and/or device.

Furthermore, specific terms used in the following description areprovided to help understanding of the present invention, and suchspecific terms may be changed into other forms without departing fromthe technological spirit of the present invention.

The following technologies may be used in a variety of wirelesscommunication systems, such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), and non-orthogonalmultiple access (NOMA). CDMA may be implemented using a radiotechnology, such as universal terrestrial radio access (UTRA) orCDMA2000. TDMA may be implemented using a radio technology, such asglobal system for Mobile communications (GSM)/general packet radioservice (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMA maybe implemented using a radio technology, such as institute of electricaland electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX),IEEE 802.20, or evolved UTRA (E-UTRA). UTRA is part of a universalmobile telecommunications system (UMTS). 3rd generation partnershipproject (3GPP) long term evolution (LTE) is part of an evolved UMTS(E-UMTS) using evolved UMTS terrestrial radio access (E-UTRA), and itadopts OFDMA in downlink and adopts SC-FDMA in uplink. LTE-advanced(LTE-A) is the evolution of 3GPP LTE.

Embodiments of the present invention may be supported by the standarddocuments disclosed in at least one of IEEE 802, 3GPP, and 3GPP2, thatis, radio access systems. That is, steps or portions that belong to theembodiments of the present invention and that are not described in orderto clearly expose the technical spirit of the present invention may besupported by the documents. Furthermore, all terms disclosed in thisdocument may be described by the standard documents.

In order to more clarify a description, IEEE 802.11 system is chieflydescribed, but the technical characteristics of the present inventionare not limited thereto.

General System

FIG. 1 is a diagram showing an example of an IEEE 802.11 system to whichan embodiment of the present invention may be applied.

The IEEE 802.11 configuration may include a plurality of elements. Theremay be provided a wireless communication system supporting transparentstation (STA) mobility for a higher layer through an interaction betweenthe elements. A basic service set (BSS) may correspond to a basicconfiguration block in an IEEE 802.11 system.

FIG. 1 illustrates that three BSSs BSS 1 to BSS 3 are present and twoSTAs (e.g., an STA 1 and an STA 2 are included in the BSS 1, an STA 3and an STA 4 are included in the BSS 2, and an STA 5 and an STA 6 areincluded in the BSS 3) are included as the members of each BSS.

In FIG. 1, an ellipse indicative of a BSS may be interpreted as beingindicative of a coverage area in which STAs included in thecorresponding BSS maintain communication. Such an area may be called abasic service area (BSA). When an STA moves outside the BSA, it isunable to directly communicate with other STAs within the correspondingBSA.

In the IEEE 802.11 system, the most basic type of a BSS is anindependent a BSS (IBSS). For example, an IBSS may have a minimum formincluding only two STAs. Furthermore, the BSS 3 of FIG. 1 which is thesimplest form and from which other elements have been omitted maycorrespond to a representative example of the IBSS. Such a configurationmay be possible if STAs can directly communicate with each other.Furthermore, a LAN of such a form is not previously planned andconfigured, but may be configured when it is necessary. This may also becalled an ad-hoc network.

When an STA is powered off or on or an STA enters into or exits from aBSS area, the membership of the STA in the BSS may be dynamicallychanged. In order to become a member of a BSS, an STA may join the BSSusing a synchronization process. In order to access all of services in aBSS-based configuration, an STA needs to be associated with the BSS.Such association may be dynamically configured, and may include the useof a distribution system service (DSS).

In an 802.11 system, the distance of a direct STA-to-STA may beconstrained by physical layer (PHY) performance. In any case, the limitof such a distance may be sufficient, but communication between STAs ina longer distance may be required, if necessary. In order to supportextended coverage, a distribution system (DS) may be configured.

The DS means a configuration in which BSSs are interconnected. Morespecifically, a BSS may be present as an element of an extended form ofa network including a plurality of BSSs instead of an independent BSS asin FIG. 1.

The DS is a logical concept and may be specified by the characteristicsof a distribution system medium (DSM). In the IEEE 802.11 standard, awireless medium (WM) and a distribution system medium (DSM) arelogically divided. Each logical medium is used for a different purposeand used by a different element. In the definition of the IEEE 802.11standard, such media are not limited to the same one and are also notlimited to different ones. The flexibility of the configuration (i.e., aDS configuration or another network configuration) of an IEEE 802.11system may be described in that a plurality of media is logicallydifferent as described above. That is, an IEEE 802.11 systemconfiguration may be implemented in various ways, and a correspondingsystem configuration may be independently specified by the physicalcharacteristics of each implementation example.

The DS can support a mobile device by providing the seamless integrationof a plurality of BSSs and providing logical services required to handlean address to a destination.

An AP means an entity which enables access to a DS through a WM withrespect to associated STAs and has the STA functionality. The movementof data between a BSS and the DS can be performed through an AP. Forexample, each of the STA 2 and the STA 3 of FIG. 1 has the functionalityof an STA and provides a function which enables associated STAs (e.g.,the STA 1 and the STA 4) to access the DS. Furthermore, all of APsbasically correspond to an STA, and thus all of the APs are entitiescapable of being addressed. An address used by an AP for communicationon a WM and an address used by an AP for communication on a DSM may notneed to be necessarily the same.

Data transmitted from one of STAs, associated with an AP, to the STAaddress of the AP may be always received by an uncontrolled port andprocessed by an IEEE 802.1X port access entity. Furthermore, when acontrolled port is authenticated, transmission data (or frame) may bedelivered to a DS.

A wireless network having an arbitrary size and complexity may include aDS and BSSs. In an IEEE 802.11 system, a network of such a method iscalled an extended service set (ESS) network. The ESS may correspond toa set of BSSs connected to a single DS. However, the ESS does notinclude a DS. The ESS network is characterized in that it looks like anIBSS network in a logical link control (LLC) layer. STAs included in theESS may communicate with each other. Mobile STAs may move from one BSSto the other BSS (within the same ESS) in a manner transparent to theLLC layer.

In an IEEE 802.11 system, the relative physical positions of BSSs inFIG. 1 are not assumed, and the following forms are all possible.

More specifically, BSSs may partially overlap, which is a form commonlyused to provide consecutive coverage. Furthermore, BSSs may not bephysically connected, and logically there is no limit to the distancebetween BSSs. Furthermore, BSSs may be placed in the same positionphysically and may be used to provide redundancy. Furthermore, one (orone or more) IBSS or ESS networks may be physically present in the samespace as one or more ESS networks. This may correspond to an ESS networkform if an ad-hoc network operates at the position in which an ESSnetwork is present, if IEEE 802.11 networks that physically overlap areconfigured by different organizations, or if two or more differentaccess and security policies are required at the same position.

In a WLAN system, an STA is an apparatus operating in accordance withthe medium access control (MAC)/PHY regulations of IEEE 802.11. An STAmay include an AP STA and a non-AP STA unless the functionality of theSTA is not individually different from that of an AP. In this case,assuming that communication is performed between an STA and an AP, theSTA may be interpreted as being a non-AP STA. In the example of FIG. 1,the STA 1, the STA 4, the STA 5, and the STA 6 correspond to non-APSTAs, and the STA 2 and the STA 3 correspond to AP STAs.

A non-AP STA corresponds to an apparatus directly handled by a user,such as a laptop computer or a mobile phone. In the followingdescription, a non-AP STA may also be called a wireless device, aterminal, user equipment (UE), a mobile station (MS), a mobile terminal,a wireless terminal, a wireless transmit/receive unit (WTRU), a networkinterface device, a machine-type communication (MTC) device, amachine-to-machine (M2M) device or the like.

Furthermore, an AP is a concept corresponding to a base station (BS), anode-B, an evolved Node-B (eNB), a base transceiver system (BTS), afemto BS or the like in other wireless communication fields.

Hereinafter, in this specification, downlink (DL) means communicationfrom an AP to a non-AP STA. Uplink (UL) means communication from anon-AP STA to an AP. In DL, a transmitter may be part of an AP, and areceiver may be part of a non-AP STA. In UL, a transmitter may be partof a non-AP STA, and a receiver may be part of an AP.

Physical Protocol Data Unit (PPDU) Format

A PPDU means a data block generated in the physical layer. A PPDU formatis described below based on an IEEE 802.11 a WLAN system to which anembodiment of the present invention may be applied.

FIG. 2 illustrates a non-HT format PPDU and an HT format PPDU in awireless communication system to which an embodiment of the presentinvention may be applied.

FIG. 2(a) illustrates a non-HT format PPDU for supporting IEEE 802.11a/gsystems. The non-HT PPDU may also be called a legacy PPDU.

Referring to FIG. 2(a), the non-HT format PPDU is configured to includea legacy format preamble, including a legacy (or non-HT) short trainingfield (L-STF), a legacy (or non-HT) long training field (L-LTF), and alegacy (or non-HT) signal (L-SIG) field, and a data field.

The L-STF may include a short training orthogonal frequency divisionmultiplexing symbol (OFDM). The L-STF may be used for frame timingacquisition, automatic gain control (AGC), diversity detection, andcoarse frequency/time synchronization.

The L-LTF may include a long training OFDM symbol. The L-LTF may be usedfor fine frequency/time synchronization and channel estimation.

The L-SIG field may be used to send control information for thedemodulation and decoding of the data field.

The L-SIG field may include a rate field of four bits, a reserved fieldof 1 bit, a length field of 12 bits, a parity bit of 1 bit, and a signaltail field of 6 bits.

The rate field includes transfer rate information, and the length fieldindicates the number of octets of a PSDU.

FIG. 2(b) illustrates an HT mixed format PPDU for supporting both anIEEE 802.11n system and IEEE 802.11a/g system.

Referring to FIG. 2(b), the HT mixed format PPDU is configured toinclude a legacy format preamble including an L-STF, an L-LTF, and anL-SIG field, an HT format preamble including an HT-signal (HT-SIG)field, a HT short training field (HT-STF), and a HT long training field(HT-LTF), and a data field.

The L-STF, the L-LTF, and the L-SIG field mean legacy fields forbackward compatibility and are the same as those of the non-HT formatfrom the L-STF to the L-SIG field. An L-STA may interpret a data fieldthrough an L-LTF, an L-LTF, and an L-SIG field although it receives anHT mixed PPDU. In this case, the L-LTF may further include informationfor channel estimation to be performed by an HT-STA in order to receivethe HT mixed PPDU and to demodulate the L-SIG field and the HT-SIGfield.

An HT-STA may be aware of an HT mixed format PPDU using the HT-SIG fieldsubsequent to the legacy fields, and may decode the data field based onthe HT mixed format PPDU.

The HT-LTF may be used for channel estimation for the demodulation ofthe data field. IEEE 802.11n supports single user multi-input andmulti-output (SU-MIMO) and thus may include a plurality of HT-LTFs forchannel estimation with respect to each of data fields transmitted in aplurality of spatial streams.

The HT-LTF may include a data HT-LTF used for channel estimation for aspatial stream and an extension HT-LTF additionally used for fullchannel sounding. Accordingly, a plurality of HT-LTFs may be the same asor greater than the number of transmitted spatial streams.

In the HT mixed format PPDU, the L-STF, the L-LTF, and the L-SIG fieldsare first transmitted so that an L-STA can receive the L-STF, the L-LTF,and the L-SIG fields and obtain data. Thereafter, the HT-SIG field istransmitted for the demodulation and decoding of data transmitted for anHT-STA.

An L-STF, an L-LTF, L-SIG, and HT-SIG fields are transmitted withoutperforming beamforming up to an HT-SIG field so that an L-STA and anHT-STA can receive a corresponding PPDU and obtain data. In an HT-STF,an HT-LTF, and a data field that are subsequently transmitted, radiosignals are transmitted through precoding. In this case, an HT-STF istransmitted so that an STA receiving a corresponding PPDU by performingprecoding may take into considerate a portion whose power is varied byprecoding, and a plurality of HT-LTFs and a data field are subsequentlytransmitted.

Table 1 below illustrates the HT-SIG field.

TABLE 1 Field Bit Description MCS 7 Indicate a modulation and codingscheme CBW 20/40 1 Set to “0” if a CBW is 20 MHz or 40 MHz orupper/lower Set to “1” if a CBW is 40 MHz HT length 16 Indicate thenumber of data octets within a PSDU Smoothing 1 Set to “1” if channelsmoothing is recommended Set to “0” if channel estimation is recommendedunsmoothingly for each carrier Not-sounding 1 Set to “0” if a PPDU is asounding PPDU Set to “1” if a PPDU is not a sounding PPDU Reserved 1 Setto “1” Aggregation 1 Set to “1” if a PPDU includes an A-MPDU Set to “0”if not Space-time 2 Indicate a difference between the number ofspace-time streams (NSTS) block coding and the number of spatial streams(NSS) indicated by an MCS (STBC) Set to “00” if an STBC is not used FECcoding 1 Set to “1” if low-density parity check (LDPC) is used Set to“0” if binary convolutional code (BCC) is used Short GI 1 Set to “1” ifa short guard interval (GI) is used after HT training Set to “0” if notNumber of 2 Indicate the number of extension spatial streams (NESSs)extension spatial Set to “0” if there is no NESS streams Set to “1” ifthe number of NESSs is 1 Set to “2” if the number of NESSs is 2 Set to“3” if the number of NESSs is 3 CRC 8 Include CRS for detecting an errorof a PPDU on the receiver side Tail bits 6 Used to terminate the trellisof a convolutional decoder Set to “0”

FIG. 2(c) illustrates an HT-green field format PPDU (HT-GF format PPDU)for supporting only an IEEE 802.11n system.

Referring to FIG. 2(c), the HT-GF format PPDU includes an HT-GF-STF, anHT-LTF1, an HT-SIG field, a plurality of HT-LTF2s, and a data field.

The HT-GF-STF is used for frame timing acquisition and AGC.

The HT-LTF1 is used for channel estimation.

The HT-SIG field is used for the demodulation and decoding of the datafield.

The HT-LTF2 is used for channel estimation for the demodulation of thedata field. Likewise, an HT-STA uses SU-MIMO. Accordingly, a pluralityof the HT-LTF2s may be configured because channel estimation isnecessary for each of data fields transmitted in a plurality of spatialstreams.

The plurality of HT-LTF2s may include a plurality of data HT-LTFs and aplurality of extension HT-LTFs like the HT-LTF of the HT mixed PPDU.

In FIGS. 2(a) to (c), the data field is a payload and may include aservice field, a scrambled PSDU (PSDU) field, tail bits, and paddingbits. All of the bits of the data field are scrambled.

FIG. 2(d) illustrates a service field included in the data field. Theservice field has 16 bits. The 16 bits are assigned No. 0 to No. 15 andare sequentially transmitted from the No. 0 bit. The No. 0 bit to theNo. 6 bit are set to 0 and are used to synchronize a descrambler withina reception stage.

An IEEE 802.11ac WLAN system supports the transmission of a DLmulti-user multiple input multiple output (MU-MIMO) method in which aplurality of STAs accesses a channel at the same time in order toefficiently use a radio channel. In accordance with the MU-MIMOtransmission method, an AP may simultaneously transmit a packet to oneor more STAs that have been subjected to MIMO pairing.

Downlink multi-user transmission (DL MU transmission) means a technologyin which an AP transmits a PPDU to a plurality of non-AP STAs throughthe same time resources using one or more antennas.

Hereinafter, an MU PPDU means a PPDU which delivers one or more PSDUsfor one or more STAs using the MU-MIMO technology or the OFDMAtechnology. Furthermore, an SU PPDU means a PPDU having a format inwhich only one PSDU can be delivered or which does not have a PSDU.

For MU-MIMO transmission, the size of control information transmitted toan STA may be relatively larger than the size of 802.11n controlinformation. Control information additionally required to supportMU-MIMO may include information indicating the number of spatial streamsreceived by each STA and information related to the modulation andcoding of data transmitted to each STA may correspond to the controlinformation, for example.

Accordingly, when MU-MIMO transmission is performed to provide aplurality of STAs with a data service at the same time, the size oftransmitted control information may be increased according to the numberof STAs which receive the control information.

In order to efficiently transmit the control information whose size isincreased as described above, a plurality of pieces of controlinformation required for MU-MIMO transmission may be divided into twotypes of control information: common control information that isrequired for all of STAs in common and dedicated control informationindividually required for a specific STA, and may be transmitted.

FIG. 3 illustrates a VHT format PPDU in a wireless communication systemto which an embodiment of the present invention may be applied.

FIG. 3(a) illustrates a VHT format PPDU for supporting an IEEE 802.11acsystem.

Referring to FIG. 3(a), the VHT format PPDU is configured to include alegacy format preamble including an L-STF, an L-LTF, and an L-SIG field,a VHT format preamble including a VHT-signal-A (VHT-SIG-A) field, a VHTshort training field (VHT-STF), a VHT long training field (VHT-LTF), anda VHT-signal-B (VHT-SIG-B) field, and a data field.

The L-STF, the L-LTF, and the L-SIG field mean legacy fields forbackward compatibility and have the same formats as those of the non-HTformat. In this case, the L-LTF may further include information forchannel estimation which will be performed in order to demodulate theL-SIG field and the VHT-SIG-A field.

The L-STF, the L-LTF, the L-SIG field, and the VHT-SIG-A field may berepeated in a 20 MHz channel unit and transmitted. For example, when aPPDU is transmitted through four 20 MHz channels (i.e., an 80 MHzbandwidth), the L-STF, the L-LTF, the L-SIG field, and the VHT-SIG-Afield may be repeated every 20 MHz channel and transmitted.

A VHT-STA may be aware of the VHT format PPDU using the VHT-SIG-A fieldsubsequent to the legacy fields, and may decode the data field based onthe VHT-SIG-A field.

In the VHT format PPDU, the L-STF, the L-LTF, and the L-SIG field arefirst transmitted so that even an L-STA can receive the VHT format PPDUand obtain data. Thereafter, the VHT-SIG-A field is transmitted for thedemodulation and decoding of data transmitted for a VHT-STA.

The VHT-SIG-A field is a field for the transmission of controlinformation that is common to a VHT STAs that are MIMO-paired with anAP, and includes control information for interpreting the received VHTformat PPDU.

The VHT-SIG-A field may include a VHT-SIG-A1 field and a VHT-SIG-A2field.

The VHT-SIG-A1 field may include information about a channel bandwidth(BW) used, information about whether space time block coding (STBC) isapplied or not, a group identifier (ID) for indicating a group ofgrouped STAs in MU-MIMO, information about the number of streams used(the number of space-time streams (NSTS)/part association identifier(AID), and transmit power save forbidden information. In this case, thegroup ID means an identifier assigned to a target transmission STA groupin order to support MU-MIMO transmission, and may indicate whether thepresent MIMO transmission method is MU-MIMO or SU-MIMO.

Table 2 illustrates the VHT-SIG-A1 field.

TABLE 2 FIELD BIT DESCRIPTION BW 2 Set to “0” if a BW is 20 MHz Set to“1” if a BW is 40 MHz Set to “2” if a BW is 80 MHz Set to “3” if a BW is160 MHz or 80 + 80 MHz Reserved 1 STBC 1 In the case of a VHT SU PPDU:Set to “1” if STBC is used Set to “0” if not In the case of a VHT MUPPDU: Set to “0” group ID 6 Indicate a group ID “0” or “63” indicates aVHT SU PPDU, but indicates a VHT MU PPDU if not NSTS/Partial AID 12 Inthe case of a VHT MU PPDU, divide into 4 user positions “p” each havingthree bits “0” if a space-time stream is 0 “1” if a space-time stream is1 “2” if a space-time stream is 2 “3” if a space-time stream is 3 “4” ifa space-time stream is 4 In the case of a VHT SU PPDU, Upper 3 bits areset as follows: “0” if a space-time stream is 1 “1” if a space-timestream is 2 “2” if a space-time stream is 3 “3” if a space-time streamis 4 “4” if a space-time stream is 5 “5” if a space-time stream is 6 “6”if a space-time stream is 7 “7” if a space-time stream is 8 Lower 9 bitsindicate a partial AID. TXOP_PS_NOT_ 1 Set to “0” if a VHT AP permits anon-AP VHT STA to switch ALLOWED to power save mode during transmissionopportunity (TXOP) Set to “1” if not In the case of a VHT PPDUtransmitted by a non-AP VHT STA Set to “1” Reserved 1

The VHT-SIG-A2 field may include information about whether a short guardinterval (GI) is used or not, forward error correction (FEC)information, information about a modulation and coding scheme (MCS) fora single user, information about the type of channel coding for multipleusers, beamforming-related information, redundancy bits for cyclicredundancy checking (CRC), the tail bits of a convolutional decoder andso on.

Table 3 illustrates the VHT-SIG-A2 field.

TABLE 3 FIELD BIT DESCRIPTION Short GI 1 Set to “0” if a short GI is notused in a data field Set to “1” if a short GI is used in a data fieldShort GI 1 Set to “1” if a short GI is used and an extra symbol isrequired disambiguation for the payload of a PPDU Set to “0” if an extrasymbol is not required SU/MU coding 1 In the case of a VHT SU PPDU: Setto “0” in the case of binary convolutional code (BCC) Set to “1” in thecase of low-density parity check (LDPC) In the case of a VHT MU PPDU:Indicate coding used if the NSTS field of a user whose user position is“0” is not “0” Set to “0” in the case of BCC Set to “1” in the case ofPDPC Set to “1” as a reserved field if the NSTS field of a user whoseuser position is “0” is “0” LDPC Extra OFDM 1 Set to “1” if an extraOFDM symbol is required due to an PDPC symbol PPDU encoding procedure(in the case of a SU PPDU) or the PPDU encoding procedure of at leastone PDPC user (in the case of a VHT MU PPDU) Set to “0” if not SU VHTMCS/MU 4 In the case of a VHT SU PPDU: coding Indicate a VHT-MCS indexIn the case of a VHT MU PPDU: Indicate coding for user positions “1” to“3” sequentially from upper bits Indicate coding used if the NSTS fieldof each user is not “1” Set to “0” in the case of BCC Set to “1” in thecase of LDPC Set to “1” as a reserved field if the NSTS field of eachuser is “0” Beamformed 1 In the case of a VHT SU PPDU: Set to “1” if abeamforming steering matrix is applied to SU transmission Set to “0” ifnot In the case of a VHT MU PPDU: Set to “1” as a reserved fieldReserved 1 CRC 8 Include CRS for detecting an error of a PPDU on thereceiver side Tail 6 Used to terminate the trellis of a convolutionaldecoder Set to “0”

The VHT-STF is used to improve AGC estimation performance in MIMOtransmission.

The VHT-LTF is used for a VHT-STA to estimate an MIMO channel. Since aVHT WLAN system supports MU-MIMO, the VHT-LTF may be configured by thenumber of spatial streams through which a PPDU is transmitted.Additionally, if full channel sounding is supported, the number ofVHT-LTFs may be increased.

The VHT-SIG-B field includes dedicated control information which isnecessary for a plurality of MU-MIMO-paired VHT-STAs to receive a PPDUand to obtain data. Accordingly, only when common control informationincluded in the VHT-SIG-A field indicates that a received PPDU is forMU-MIMO transmission, a VHT-STA may be designed to decode the VHT-SIG-Bfield. In contrast, if common control information indicates that areceived PPDU is for a single VHT-STA (including SU-MIMO), an STA may bedesigned to not decode the VHT-SIG-B field.

The VHT-SIG-B field includes a VHT-SIG-B length field, a VHT-MCS field,a reserved field, and a tail field.

The VHT-SIG-B length field indicates the length of an A-MPDU (prior toend-of-frame (EOF) padding). The VHT-MCS field includes informationabout the modulation, encoding, and rate-matching of each VHT-STA.

The size of the VHT-SIG-B field may be different depending on the type(MU-MIMO or SU-MIMO) of MIMO transmission and a channel bandwidth usedfor PPDU transmission.

FIG. 3(b) illustrates a VHT-SIG-B field according to a PPDU transmissionbandwidth.

Referring to FIG. 3(b), in 40 MHz transmission, VHT-SIG-B bits arerepeated twice. In 80 MHz transmission, VHT-SIG-B bits are repeated fourtimes, and padding bits set to 0 are attached.

In 160 MHz transmission and 80+80 MHz transmission, first, VHT-SIG-Bbits are repeated four times as in the 80 MHz transmission, and paddingbits set to 0 are attached. Furthermore, a total of the 117 bits isrepeated again.

In a system supporting MU-MIMO, in order to transmit PPDUs having thesame size to STAs paired with an AP, information indicating the size ofthe bits of a data field forming the PPDU and/or information indicatingthe size of bit streams forming a specific field may be included in theVHT-SIG-A field.

In this case, an L-SIG field may be used to effectively use a PPDUformat. A length field and a rate field which are included in the L-SIGfield and transmitted so that PPDUs having the same size are transmittedto all of STAs may be used to provide required information. In thiscase, additional padding may be required in the physical layer becausean MAC protocol data unit (MPDU) and/or an aggregate MAC PDU (A-MPDU)are set based on the bytes (or octets) of the MAC layer.

In FIG. 3, the data field is a payload and may include a service field,a scrambled PSDU, tail bits, and padding bits.

An STA needs to determine the format of a received PPDU because severalformats of PPDUs are mixed and used as described above.

In this case, the meaning that a PPDU (or a PPDU format) is determinedmay be various. For example, the meaning that a PPDU is determined mayinclude determining whether a received PPDU is a PPDU capable of beingdecoded (or interpreted) by an STA. Furthermore, the meaning that a PPDUis determined may include determining whether a received PPDU is a PPDUcapable of being supported by an STA. Furthermore, the meaning that aPPDU is determined may include determining that information transmittedthrough a received PPDU is which information.

This will be described in more detail below with reference to thedrawings.

FIG. 4 illustrates constellation diagrams for classifying a PPDU formatin a wireless communication system to which the present invention may beapplied.

FIG. 4(a) illustrates a constellation for the L-SIG field included inthe non-HT format PPDU, FIG. 4(b) illustrates a phase rotation forHT-mixed format PPDU detection, and FIG. 4(c) illustrates a phaserotation for VHT format PPDU detection.

In order for an STA to classify a PPDU as a non-HT format PPDU, HT-GFformat PPDU, HT-mixed format PPDU, or VHT format PPDU, the phases ofconstellations of the L-SIG field and of the OFDM symbols, which aretransmitted following the L-SIG field, are used. That is, the STA mayclassify a PDDU format based on the phases of constellations of theL-SIG field of a received PPDU and/or of the OFDM symbols, which aretransmitted following the L-SIG field.

Referring to FIG. 4(a), the OFDM symbols of the L-SIG field use BPSK(Binary Phase Shift Keying).

To begin with, in order to classify a PPDU as an HT-GF format PPDU, theSTA, upon detecting a first SIG field from a received PPDU, determineswhether this first SIG field is an L-SIG field or not. That is, the STAattempts to perform decoding based on the constellation illustrated in(a) of FIG. 5. If the STA fails in decoding, the corresponding PPDU maybe classified as the HT-GF format PPDU.

Next, in order to distinguish the non-HT format PPDU, HT-mixed formatPPDU, and VHT format PPDU, the phases of constellations of the OFDMsymbols transmitted following the L-SIG field may be used. That is, themethod of modulation of the OFDM symbols transmitted following the L-SIGfield may vary, and the STA may classify a PPDU format based on themethod of modulation of fields coming after the L-SIG field of thereceived PPDU.

Referring to 4(b), in order to classify a PPDU as an HT-mixed formatPPDU, the phases of two OFDM symbols transmitted following the L-SIGfield in the HT-mixed format PPDU may be used.

More specifically, both the phases of OFDM symbols #1 and #2corresponding to the HT-SIG field, which is transmitted following theL-SIG field, in the HT-mixed format PPDU are rotated counterclockwise by90 degrees. That is, the OFDM symbols #1 and #2 are modulated by QBPSK(Quadrature Binary Phase Shift Keying). The QBPSK constellation may be aconstellation which is rotated counterclockwise by 90 degrees based onthe BPSK constellation.

An STA attempts to decode the first and second OFDM symbolscorresponding to the HT-SIG field transmitted after the L-SIG field ofthe received PDU, based on the constellations illustrated in FIG. 5(b).If the STA succeeds in decoding, the corresponding PPDU may beclassified as an HT format PPDU.

Next, in order to distinguish the non-HT format PPDU and the VHT formatPPDU, the phases of constellations of the OFDM symbols transmittedfollowing the L-SIG field may be used.

Referring to 4(c), in order to classify a PPDU as a VHT format PPDU, thephases of two OFDM symbols transmitted after the L-SIG field may be usedin the VHT format PPDU.

More specifically, the phase of the OFDM symbol #1 corresponding to theVHT-SIG-A coming after the L-SIG field in the HT format PPDU is notrotated, but the phase of the OFDM symbol #2 is rotated counterclockwiseby 90 degrees. That is, the OFDM symbol #1 is modulated by BPSK, and theOFDM symbol #2 is modulated by QBPSK.

The STA attempts to decode the first and second OFDM symbolscorresponding to the VHT-SIG field transmitted following the L-SIG fieldof the received PDU, based on the constellations illustrated in (c) ofFIG. 5. If the STA succeeds in decoding, the corresponding PPDU may beclassified as a VHT format PPDU.

On the contrary, If the STA fails in decoding, the corresponding PPDUmay be classified as a non-HT format PPDU.

MAC Frame Format

FIG. 5 illustrates a MAC frame format in an IEEE 802.11 system to whichthe present invention may be applied.

Referring to FIG. 5, the MAC frame (i.e., an MPDU) includes an MACheader, a frame body, and a frame check sequence (FCS).

The MAC Header is defined as an area, including a frame control field, aduration/ID field, an address 1 field, an address 2 field, an address 3field, a sequence control field, an address 4 field, a QoS controlfield, and an HT control field.

The frame control field contains information on the characteristics ofthe MAC frame. A more detailed description of the frame control fieldwill be given later.

The duration/ID field may be implemented to have a different valuedepending on the type and subtype of a corresponding MAC frame.

If the type and subtype of a corresponding MAC frame is a PS-poll framefor a power save (PS) operation, the duration/ID field may be configuredto include the association identifier (AID) of an STA that hastransmitted the frame. In the remaining cases, the duration/ID field maybe configured to have a specific duration value depending on the typeand subtype of a corresponding MAC frame. Furthermore, if a frame is anMPDU included in an aggregate-MPDU (A-MPDU) format, the duration/IDfield included in an MAC header may be configured to have the samevalue.

The address 1 field to the address 4 field are used to indicate a BSSID,a source address (SA), a destination address (DA), a transmittingaddress (TA) indicating the address of a transmitting STA, and areceiving address (RA) indicating the address of a receiving STA.

Meanwhile, an address field implemented as a TA field may be set as abandwidth signaling TA value. In this case, the TA field may indicatethat a corresponding MAC frame includes additional information in ascrambling sequence. The bandwidth signaling TA may be represented asthe MAC address of an STA that sends a corresponding MAC frame, butindividual/group bits included in the MAC address may be set as aspecific value (e.g., “1”).

The sequence control field is configured to include a sequence numberand a fragment number. The sequence number may indicate a sequencenumber assigned to a corresponding MAC frame. The fragment number mayindicate the number of each fragment of a corresponding MAC frame.

The QoS control field includes information related to QoS. The QoScontrol field may be included if it indicates a QoS data frame in asubtype subfield.

The HT control field includes control information related to an HTand/or VHT transmission/reception scheme. The HT control field isincluded in a control wrapper frame. Furthermore, the HT control fieldis present in a QoS data frame having an order subfield value of 1 and amanagement frame.

The frame body is defined as an MAC payload. Data to be transmitted in ahigher layer is placed in the frame body. The frame body has a varyingsize. For example, a maximum size of an MPDU may be 11454 octets, and amaximum size of a PPDU may be 5.484 ms.

The FCS is defined as an MAC footer and used for the error search of anMAC frame.

The first three fields (i.e., the frame control field, the duration/IDfield, and Address 1 field) and the last field (i.e., the FCS field)form a minimum frame format and are present in all of frames. Theremaining fields may be present only in a specific frame type.

FIG. 6 is a diagram illustrating the frame control field in the MACframe in a wireless communication system to which the present inventionmay be applied.

Referring to FIG. 6, the frame control field includes a Protocol Versionsubfield, a Type subfield, a Subtype subfield, a to DS subfield, a FromDS subfield, a More Fragments subfield, a Retry subfield, a PowerManagement subfield, a More Data subfield, a Protected Frame subfield,and an Order subfield.

The protocol version subfield may indicate the version of a WLANprotocol applied to the MAC frame.

The type subfield and the subtype subfield may be configured to indicateinformation for identifying the function of the MAC frame.

The MAC frame may include three frame types: Management frames, Controlframes, and Data frames.

Furthermore, each frame type may be subdivided into subtypes.

For example, the Control frames may include an RTS (request-to-send)frame, a CTS (clear-to-send) frame, an ACK (Acknowledgement) frame, aPS-Poll frame, a CF (contention free)-End frame, a CF-End+CF-ACK frame,a BAR (Block Acknowledgement request) frame, a BA (BlockAcknowledgement) frame, a Control Wrapper (Control+HTcontrol) frame, aVHT NDPA (Null Data Packet Announcement) frame, and a Beamforming ReportPoll frame.

The Management frames may include a Beacon frame, an ATIM (AnnouncementTraffic Indication Message) frame, a Disassociation frame, anAssociation Request/Response frame, a Reassociation Request/Responseframe, a Probe Request/Response frame, an Authentication frame, aDeauthentication frame, an Action frame, an Action No ACK frame, and aTiming Advertisement frame.

The To Ds subfield and the From DS subfield may contain informationrequired to interpret the Address 1 field through Address 4 fieldincluded in the MAC frame header. For a Control frame, the To DSsubfield and the From DS subfield may all set to ‘0’. For a Managementframe, the To DS subfield and the From DS subfield may be set to ‘1’ and‘0’, respectively, if the corresponding frame is a QoS Management frame(QMF); otherwise, the To DS subfield and the From DS subfield all may beset to ‘0’.

The More Fragments subfield may indicate whether there is a fragment tobe sent subsequent to the MAC frame. If there is another fragment of thecurrent MSDU or MMPDU, the More Fragments subfield may be set to ‘1’;otherwise, it may be set to ‘0’.

The Retry subfield may indicate whether the MAC frame is the previousMAC frame that is re-transmitted. If the MAC frame is the previous MACframe that is re-transmitted, the Retry subfield may be set to ‘1’;otherwise, it may be set to ‘0’.

The Power Management subfield may indicate the power management mode ofthe STA. If the Power Management subfield has a value of ‘1’, this mayindicate that the STA switches to power save mode.

The More Data subfield may indicate whether there is a MAC frame to beadditionally sent. If there is a MAC frame to be additionally sent, theMore Data subfield may be set to ‘1’; otherwise, it may be set to ‘0’.

The Protected Frame subfield may indicate whether a Frame Body field isencrypted or not. If the Frame Body field contains information that isprocessed by a cryptographic encapsulation algorithm, it may be set to‘1’; otherwise ‘0’.

Information contained in the above-described fields may be as defined inthe IEEE 802.11 system. The above-described fields are examples of thefields that may be included in the MAC frame but not limited to them.That is, the above-described fields may be substituted with other fieldsor further include additional fields, and not all of the fields may benecessarily included.

FIG. 7 illustrates the VHT format of an HT control field in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

Referring to FIG. 7, the HT control field may include a VHT subfield, anHT control middle subfield, an AC constraint subfield, and a reversedirection grant (RDG)/more PPDU subfield.

The VHT subfield indicates whether the HT control field has the formatof an HT control field for VHT (VHT=1) or has the format of an HTcontrol field for HT (VHT=0). In FIG. 8, it is assumed that the HTcontrol field is an HT control field for VHT (i.e., VHT=1). The HTcontrol field for VHT may be called a VHT control field.

The HT control middle subfield may be implemented to a different formatdepending on the indication of a VHT subfield. The HT control middlesubfield is described in detail later.

The AC constraint subfield indicates whether the mapped access category(AC) of a reverse direction (RD) data frame is constrained to a singleAC.

The RDG/more PPDU subfield may be differently interpreted depending onwhether a corresponding field is transmitted by an RD initiator or an RDresponder.

The RDG/More PPDU subfield may be interpreted differently depending onwhether the corresponding field is sent by an RD initiator or an RDresponder.

Assuming that a corresponding field is transmitted by an RD initiator,the RDG/more PPDU subfield is set as “1” if an RDG is present, and theRDG/more PPDU subfield is set as “0” if an RDG is not present. Assumingthat a corresponding field is transmitted by an RD responder, theRDG/more PPDU subfield is set as “1” if a PPDU including thecorresponding subfield is the last frame transmitted by the RDresponder, and the RDG/more PPDU subfield is set as “0” if another PPDUis transmitted.

As described above, the HT control middle subfield may be implemented toa different format depending on the indication of a VHT subfield.

The HT control middle subfield of an HT control field for VHT mayinclude a reserved bit subfield, a modulation and coding scheme (MCS)feedback request (MRQ) subfield, an MRQ sequence identifier(MSI)/space-time block coding (STBC) subfield, an MCS feedback sequenceidentifier (MFSI)/least significant bit (LSB) of group ID (GID-L)subfield, an MCS feedback (MFB) subfield, a most significant Bit (MSB)of group ID (GID-H) subfield, a coding type subfield, a feedbacktransmission type (FB Tx type) subfield, and an unsolicited MFBsubfield.

Table 4 illustrates a description of each subfield included in the HTcontrol middle subfield of the VHT format.

TABLE 4 SUBFIELD MEANING DEFINITION MRQ MCS request Set to “1” if MCSfeedback (solicited MFB) is not requested Set to “0” if not MSI MRQsequence An MSI subfield includes a sequence numberwithin a rangeidentifier of 0 to 6 to identify a specific request if an unsolicitedMFB subfield is set to “0” and an MRQ subfield is set to “1.” Include acompressed MSI subfield (2 bits) and an STBC indication subfield (1 bit)if an unsolicited MFB subfield is “1.” MFSI/GID-L MFB sequence AnMFSI/GID-L subfield includes the received value of an identifier/LSB ofMSI included within a frame related to MFB information if group ID anunsolicited MFB subfield is set to “0.” An MFSI/GID-L subfield includesthe lowest three bits of a group ID of a PPDU estimated by an MFB if anMFB is estimated from an MU PPDU. MFB VHT N_STS, An MFB subfieldincludes recommended MFB. MCS, BW, SNR VHT-MCS = 15, NUM_STS = 7indicates that feedback is not feedback present. GID-H MSB of group ID AGID-H subfield includes the most significant bit 3 bits of a group ID ofa PPDU whose solicited MFB has been estimated if an unsolicited MFBfield is set to “1” and MFB has been estimated from a VHT MU PPDU. Allof GID-H subfields are set to “1” if MFB is estimated from an SU PPDU.Coding Type Coding type or If an unsolicited MFB subfield is set to “1”, a coding type MFB response subfield includes the coding type (binaryconvolutional code (BCC) includes 0 and low-density parity check (LDPC)includes 1) of a frame whose solicited MFB has been estimated FB Tx TypeTransmission type An FB Tx Type subfield is set to “0” if an unsolicitedMFB of MFB response subfield is set to “1” and MFB has been estimatedfrom an unbeamformed VHT PPDU. An FB Tx Type subfield is set to “1” ifan unsolicited MFB subfield is set to “1” and MFB has been estimatedfrom a beamformed VHT PPDU. Unsolicited Unsolicited MCS Set to “1” ifMFB is a response to MRQ MFB feedback indicator Set to “0” if MFB is nota response to MRQ

Furthermore, the MFB subfield may include the number of VHT space timestreams (NUM_STS) subfield, a VHT-MCS subfield, a bandwidth (BW)subfield, and a signal to noise ratio (SNR) subfield.

The NUM_STS subfield indicates the number of recommended spatialstreams. The VHT-MCS subfield indicates a recommended MCS. The BWsubfield indicates bandwidth information related to a recommended MCS.The SNR subfield indicates an average SNR value of data subcarriers andspatial streams.

The information included in each of the aforementioned fields may complywith the definition of an IEEE 802.11 system. Furthermore, each of theaforementioned fields corresponds to an example of fields which may beincluded in an MAC frame and is not limited thereto. That is, each ofthe aforementioned fields may be substituted with another field,additional fields may be further included, and all of the fields may notbe essentially included.

Downlink (DL) MU-MIMO Frame

FIG. 8 is a diagram illustrating a DL multi-user (MU) PPDU format in awireless communication system to which an embodiment of the presentinvention may be applied.

Referring to FIG. 8, the PPDU is configured to include a preamble and adata field. The data field may include a service field, a scrambled PSDUfield, tail bits, and padding bits.

An AP may aggregate MPDUs and transmit a data frame using an aggregatedMPDU (A-MPDU) format. In this case, a scrambled PSDU field may includethe A-MPDU.

The A-MPDU includes a sequence of one or more A-MPDU subframes.

In the case of a VHT PPDU, the length of each A-MPDU subframe is amultiple of 4 octets. Accordingly, an A-MPDU may include an end-of-frame(EOF) pad of 0 to 3 octets after the last A-MPDU subframe in order tomatch the A-MPDU up with the last octet of a PSDU.

The A-MPDU subframe includes an MPDU delimiter, and an MPDU may beoptionally included after the MPDU delimiter. Furthermore, a pad octetis attached to the MPDU in order to make the length of each A-MPDUsubframe in a multiple of 4 octets other than the last A-MPDU subframewithin one A-MPDU.

The MPDU delimiter includes a reserved field, an MPDU length field, acyclic redundancy check (CRC) field, and a delimiter signature field.

In the case of a VHT PPDU, the MPDU delimiter may further include anend-of-frame (EOF) field. If an MPDU length field is 0 and an A-MPDUsubframe or A-MPDU used for padding includes only one MPDU, in the caseof an A-MPDU subframe on which a corresponding MPDU is carried, the EOFfield is set to “1.” If not, the EOF field is set to “0.”

The MPDU length field includes information about the length of the MPDU.

If an MPDU is not present in a corresponding A-MPDU subframe, the PDUlength field is set to “0.” An A-MPDU subframe in which an MPDU lengthfield has a value of “0” is used to be padded to a corresponding A-MPDUin order to match the A-MPDU up with available octets within a VHT PPDU.

The CRC field includes CRC information for an error check. The delimitersignature field includes pattern information used to search for an MPDUdelimiter.

Furthermore, the MPDU includes an MAC header, a frame body, and a framecheck sequence (FCS).

FIG. 9 is a diagram illustrating a DL multi-user (MU) PPDU format in awireless communication system to which an embodiment of the presentinvention may be applied.

In FIG. 9, the number of STAs receiving a corresponding PPDU is assumedto be 3 and the number of spatial streams allocated to each STA isassumed to be 1, but the number of STAs paired with an AP and the numberof spatial streams allocated to each STA are not limited thereto.

Referring to FIG. 9, the MU PPDU is configured to include L-TFs (i.e.,an L-STF and an L-LTF), an L-SIG field, a VHT-SIG-A field, a VHT-TFs(i.e., a VHT-STF and a VHT-LTF), a VHT-SIG-B field, a service field, oneor more PSDUs, a padding field, and a tail bit. The L-TFs, the L-SIGfield, the VHT-SIG-A field, the VHT-TFs, and the VHT-SIG-B field are thesame as those of FIG. 4, and a detailed description thereof is omitted.

Information for indicating PPDU duration may be included in the L-SIGfield. In the PPDU, PPDU duration indicated by the L-SIG field includesa symbol to which the VHT-SIG-A field has been allocated, a symbol towhich the VHT-TFs have been allocated, a field to which the VHT-SIG-Bfield has been allocated, bits forming the service field, bits forming aPSDU, bits forming the padding field, and bits forming the tail field.An STA receiving the PPDU may obtain information about the duration ofthe PPDU through information indicating the duration of the PPDUincluded in the L-SIG field.

As described above, group ID information and time and spatial streamnumber information for each user are transmitted through the VHT-SIG-A,and a coding method and MCS information are transmitted through theVHT-SIG-B. Accordingly, beamformees may check the VHT-SIG-A and theVHT-SIG-B and may be aware whether a frame is an MU MIMO frame to whichthe beamformee belongs. Accordingly, an STA which is not a member STA ofa corresponding group ID or which is a member of a corresponding groupID, but in which the number of streams allocated to the STA is “0” isconfigured to stop the reception of the physical layer to the end of thePPDU from the VHT-SIG-A field, thereby being capable of reducing powerconsumption.

In the group ID, an STA can be aware that a beamformee belongs to whichMU group and it is a user who belongs to the users of a group to whichthe STA belongs and who is placed at what place, that is, that a PPDU isreceived through which stream by previously receiving a group IDmanagement frame transmitted by a beamformer.

All of MPDUs transmitted within the VHT MU PPDU based on 802.11ac areincluded in the A-MPDU. In the data field of FIG. 18, each VHT A-MPDUmay be transmitted in a different stream.

In FIG. 9, the A-MPDUs may have different bit sizes because the size ofdata transmitted to each STA may be different.

In this case, null padding may be performed so that the time when thetransmission of a plurality of data frames transmitted by a beamformeris ended is the same as the time when the transmission of a maximuminterval transmission data frame is ended. The maximum intervaltransmission data frame may be a frame in which valid downlink data istransmitted by a beamformer for the longest time. The valid downlinkdata may be downlink data that has not been null padded. For example,the valid downlink data may be included in the A-MPDU and transmitted.Null padding may be performed on the remaining data frames other thanthe maximum interval transmission data frame of the plurality of dataframes.

For the null padding, a beamformer may fill one or more A-MPDUsubframes, temporally placed in the latter part of a plurality of A-MPDUsubframes within an A-MPDU frame, with only an MPDU delimiter fieldthrough encoding. An A-MPDU subframe having an MPDU length of 0 may becalled a null subframe.

As described above, in the null subframe, the EOF field of the MPDUdelimiter is set to “1.” Accordingly, when the EOF field set to 1 isdetected in the MAC layer of an STA on the receiving side, the receptionof the physical layer is stopped, thereby being capable of reducingpower consumption.

Block Ack Procedure

FIG. 10 is a diagram illustrating a downlink MU-MIMO transmissionprocess in a wireless communication system to which the presentinvention may be applied.

MI-MIMO in 802.11ac works only in the downlink direction from the AP toclients. A multi-user frame can be transmitted to multiple receivers atthe same time, but the acknowledgements must be transmitted individuallyin the uplink direction.

Every MPDU transmitted in a VHT MU PPDU based on 802.11ac is included inan A-MPDU, so responses to A-MPDUs within the VHT MU PPDU that are notimmediate responses to the VHT MU PPDU are transmitted in response toBAR (Block Ack Request) frames by the AP.

To begin with, the AP transmits a VHT MU PPDU (i.e., a preamble anddata) to every receiver (i.e., STA 1, STA 2, and STA 3). The VHT MU PPDUincludes VHT A-MPDUs that are to be transmitted to each STA.

Having received the VHT MU PPDU from the AP, STA 1 transmits a BA (BlockAcknowledgement) frame to the AP after an SIFS. A more detaileddescription of the BA frame will be described later.

Having received the BA from STA 1, the AP transmits a BAR (blockacknowledgement request) frame to STA 2 after an SIFS, and STA 2transmits a BA frame to the AP after an SIFS. Having received the BAframe from STA 2, the AP transmits a BAR frame to STA 3 after an SIFS,and STA 3 transmits a BA frame to the AP after an SIFS.

When this process is performed all STAs, the AP transmits the next MUPPDU to all the STAs.

ACK (Acknowledgement)/Block ACK Frames

In general, an ACK frame is used as a response to an MPDU, and a blockACK frame is used as a response to an A-MPDU.

FIG. 11 is a diagram illustrating an ACK frame in a wirelesscommunication system to which the present invention may be applied.

Referring to FIG. 11, the ACK frame consists of a Frame Control field, aDuration field, an RA field, and a FCS.

The RA field is set to the value of the Address 2 field of theimmediately preceding Data frame, Management frame, Block Ack Requestframe, Block Ack frame, or PS-Poll frame.

For ACK frames sent by non-QoS STAs, if the More Fragments subfield isset to 0 in the Frame Control field of the immediately preceding Data orManagement frame, the duration value is set to 0.

For ACK frames not sent by non-QoS STAs, the duration value is set tothe value obtained from the Duration/ID field of the immediatelypreceding Data, Management, PS-Poll, BlockAckReq, or BlockAck frameminus the time, in microseconds, required to transmit the ACK frame andits SIFS interval. If the calculated duration includes a fractionalmicrosecond, that value is rounded up to the next higher integer.

Hereinafter, the Block Ack Request frame will be discussed.

FIG. 12 is a diagram illustrating a Block Ack Request frame in awireless communication system to which the present invention may beapplied.

Referring to FIG. 12, the Block Ack Request frame consists of a FrameControl field, a Duration/ID field, an RA field, a TA field, a BARControl field, a BAR Information field, and a frame check sequence(FCS).

The RA field may be set as the address of the STA receiving the BARframe.

The TA field may be set as the address of the STA transmitting the BARframe.

The BAR Control field includes a BAR Ack Policy subfield, a Multi-TIDsubfield, a Compressed Bitmap subfield, a Reserved subfield, and a TIDInfo subfield.

Table 5 shows the BAR Control field.

TABLE 5 Subfield Bits Description BAR Ack Policy 1 Set to 0 when thesender requires immediate ACK of a data transmission. Set to 1 when thesender does not require immediate ACK of a data transmission. Multi-TID1 Indicates the type of the BAR frame depending on the values of theCompressed 1 Multi-TID subfield and Compressed Bitmap subfield. Bitmap00: Basic BA 01: Compressed BA 10: Reserved 11: Multi-TID BA Reserved 9TID_Info 4 The meaning of the TID_Info field depends on the type of theBA frame. For a Basic BA frame and a Compressed BA frame, this subfieldcontains information on TIDs for which a BA frame is required. For aMulti-TID BA frame, this subfield contains the number of TIDs.

The BAR Information field contains different information depending onthe type of the BAR frame. This will be described with reference to FIG.13.

FIG. 13 is a diagram illustrating the BAR Information field of a BlockAck Request frame in a wireless communication system to which thepresent invention may be applied.

FIG. 13(a) illustrates the BAR Information field of Basic BAR andCompressed BAR frames, and FIG. 13(b) illustrates the BAR Informationfield of a Multi-TID BAR frame.

Referring to FIG. 13(a), for the Basic BAR and Compressed BAR frames,the BAR Information field includes a Block Ack Starting Sequence Controlsubfield.

The Block Ack Starting Sequence Control subfield includes a FragmentNumber subfield and a Starting Sequence Number subfield.

The Fragment Number subfield is set to 0.

For the Basic BAR frame, the Starting Sequence Number subfield containsthe sequence number of the first MSDU for which the corresponding BARframe is sent. For the Compressed BAR frame, the Starting SequenceControl subfield contains the sequence number of the first MSDU orA-MSDU for which the corresponding BAR frame is sent.

Referring to FIG. 13(b), for the Multi-TID BAR frame, the BARInformation field includes a Per TID Info subfield and a Block AckStarting Sequence Control subfield, which are repeated for each TID.

The Per TID Info subfield includes a Reserved subfield and a TID Valuesubfield. The TID Value subfield contains a TID value.

As described above, the Block Ack Starting Sequence Control subfieldincludes fragment Number and Starting Sequence Number subfields. TheFragment Number subfield is set to 0. The Starting Sequence Controlsubfield contains the sequence number of the first MSDU or A-MSDU forwhich the corresponding BAR frame is sent.

FIG. 14 is a diagram illustrating a Block Ack frame in a wirelesscommunication system to which the present invention may be applied.

Referring to FIG. 14, the Block Ack (BA) frame consists of a FrameControl field, a Duration/ID field, an RA field, a TA field, a BAControl field, a BA Information field, and a Frame Check Sequence (FCS).

The RA field may be set as the address of the STA requesting the BAframe.

The TA field may be set as the address of the STA transmitting the BAframe.

The BA Control field includes a BA Ack Policy subfield, a Multi-TIDsubfield, a Compressed Bitmap subfield, a Reserved subfield, and a TIDInfo subfield.

Table 6 shows the BA Control field.

TABLE 6 SUBFIELD BITS DESCRIPTION BA Ack Policy 1 Set to 0 when thesender requires immediate ACK of a data transmission. Set to 1 when thesender does not require immediate ACK of a data transmission. Multi-TID1 Indicates the type of the BA frame depending on the values of theCompressed 1 Multi-TID subfield and Compressed Bitmap subfield. Bitmap00: Basic BA 01: Compressed BA 10: Reserved 11: Multi-TID BA Reserved 9TID_Info 4 The meaning of the TID_Info field depends on the type of theBA frame. For a Basic BA frame and a Compressed BA frame, this subfieldcontains information on TIDs for which a BA frame is required. For aMulti-TID BA frame, this subfield contains the number of TIDs.

The BA Information field contains different information depending on thetype of the BA frame. This will be described with reference to FIG. 15.

FIG. 15 is a diagram illustrating the BA Information field of a BlockAck frame in a wireless communication system to which the presentinvention may be applied.

FIG. 15(a) illustrates the BA Information field of a Basic BA frame,FIG. 15(b) illustrates the BA Information field of a Compressed BARframe, and FIG. 15 (c) illustrates the BA Information field of aMulti-TID BA frame.

Referring to FIG. 15(a), for the Basic BA frame, the BA Informationfield includes a Block Ack Starting Sequence Control subfield and aBlock Ack Bitmap subfield.

As described above, the Block Ack Starting Sequence Control subfieldincludes a Fragment Number subfield and a Starting Sequence Numbersubfield.

The Fragment Number subfield is set to 0.

The Starting Sequence Number subfield contains the sequence number ofthe first MSDU for which the corresponding BA frame is sent, and is setto the same value as the immediately preceding Basic BAR frame.

The Block Ack Bitmap subfield is 128 octets in length and is used toindicate the received status of a maximum of 64 MSDUs. If a bit of theBlock Ack Bitmap subfield has a value of ‘1’, it indicates thesuccessful reception of a single MSDU corresponding to that bitposition, and if a bit of the Block Ack Bitmap subfield has a value of‘0’, it indicates the unsuccessful reception of a single MSDUcorresponding to that bit position.

Referring to FIG. 15(b), for the Compressed BA frame, the BA Informationfield includes a Block Ack Starting Sequence Control subfield and aBlock Ack Bitmap subfield.

As described above, the Block Ack Starting Sequence Control subfieldincludes a Fragment Number subfield and a Starting Sequence Numbersubfield.

The Fragment Number subfield is set to 0.

The Starting Sequence Number subfield contains the sequence number ofthe first MSDU or A-MSDU for which the corresponding BA frame is sent,and is set to the same value as the immediately preceding Basic BARframe.

The Block Ack Bitmap subfield is 8 octets in length and is used toindicate the received status of a maximum of 64 MSDUs and A-MSDU. If abit of the Block Ack Bitmap subfield has a value of ‘1’, it indicatesthe successful reception of a single MSDU or A-MSDU corresponding tothat bit position, and if a bit of the Block Ack Bitmap subfield has avalue of ‘0’, it indicates the unsuccessful reception of a single MSDUor A-MSDU corresponding to that bit position.

Referring to FIG. 15(c), for the Multi-TID BA frame, the BA Informationfield includes a Per TID Info subfield and a Block Ack Starting SequenceControl subfield, which are repeated for each TID in order of increasingTID.

The Per TID Info subfield includes a Reserved subfield and a TID Valuesubfield. The TID Value subfield contains a TID value.

As described above, the Block Ack Starting Sequence Control subfieldincludes fragment Number and Starting Sequence Number subfields. TheFragment Number subfield is set to 0. The Starting Sequence Controlsubfield contains the sequence number of the first MSDU or A-MSDU forwhich the corresponding BA frame is sent.

The Block Ack Bitmap subfield is 8 octets in length. If a bit of theBlock Ack Bitmap subfield has a value of ‘1’, it indicates thesuccessful reception of a single MSDU or A-MSDU corresponding to thatbit position, and if a bit of the Block Ack Bitmap subfield has a valueof ‘0’, it indicates the unsuccessful reception of a single MSDU orA-MSDU corresponding to that bit position.

UL Multiple User (MU) Transmission Method

A new frame format and numerology for an 802.11ax system, that is, thenext-generation WLAN system, are actively discussed in the situation inwhich vendors of various fields have lots of interests in thenext-generation Wi-Fi and a demand for high throughput and quality ofexperience (QoE) performance improvement are increased after 802.11ac.

IEEE 802.11ax is one of WLAN systems recently and newly proposed as thenext-generation WLAN systems for supporting a higher data rate andprocessing a higher user load, and is also called a so-called highefficiency WLAN (HEW).

An IEEE 802.11ax WLAN system may operate in a 2.4 GHz frequency band anda 5 GHz frequency band like the existing WLAN systems. Furthermore, theIEEE 802.11ax WLAN system may also operate in a higher 60 GHz frequencyband.

In the IEEE 802.11ax system, an FFT size four times larger than that ofthe existing IEEE 802.11 OFDM systems (e.g., IEEE 802.11a, 802.11n, and802.11ac) may be used in each bandwidth for average throughputenhancement and outdoor robust transmission for inter-symbolinterference. This is described below with reference to relateddrawings.

Hereinafter, in a description of an HE format PPDU according to anembodiment of the present invention, the descriptions of theaforementioned non-HT format PPDU, HT mixed format PPDU, HT-green fieldformat PPDU and/or VHT format PPDU may be reflected into the descriptionof the HE format PPDU although they are not described otherwise.

FIG. 16 is a diagram illustrating a high efficiency (HE) format PPDUaccording to an embodiment of the present invention.

FIG. 16(a) illustrates a schematic configuration of the HE format PPDU,and FIGS. 25(b) to (d) illustrate more detailed configurations of the HEformat PPDU.

Referring to FIG. 16(a), the HE format PPDU for an HEW may basicallyinclude a legacy part (L-part), an HE-part, and an HE-data field.

The L-part includes an L-STF, an L-LTF, and an L-SIG field as in a formmaintained in the existing WLAN system. The L-STF, the L-LTF, and theL-SIG field may be called a legacy preamble.

The HE-part is a part newly defined for the 802.11ax standard and mayinclude an HE-STF, a HE-SIG field, and an HE-LTF. In FIG. 25(a), thesequence of the HE-STF, the HE-SIG field, and the HE-LTF is illustrated,but the HE-STF, the HE-SIG field, and the HE-LTF may be configured in adifferent sequence. Furthermore, the HE-LTF may be omitted. Not only theHE-STF and the HE-LTF, but the HE-SIG field may be commonly called anHE-preamble.

The HE-SIG may include information (e.g., OFDMA, UL MU MIMO, andimproved MCS) for decoding the HE-data field.

The L-part and the HE-part may have different fast Fourier transform(FFT) sizes (i.e., different subcarrier spacing) and use differentcyclic prefixes (CPs).

In an 802.11ax system, an FFT size four times (4×) larger than that of alegacy WLAN system may be used. That is, the L-part may have a 1× symbolstructure, and the HE-part (more specifically, HE-preamble and HE-data)may have a 4× symbol structure. In this case, the FFT of a 1×, 2×, or 4×size means a relative size for a legacy WLAN system (e.g., IEEE 802.11a,802.11n, and 802.11ac).

For example, if the sizes of FFTs used in the L-part are 64, 128, 256,and 512 in 20 MHz, 40 MHz, 80 MHz, and 160 MHz, respectively, the sizesof FFTs used in the HE-part may be 256, 512, 1024, and 2048 in 20 MHz,40 MHz, 80 MHz, and 160 MHz, respectively.

If an FFT size is larger than that of a legacy WLAN system as describedabove, subcarrier frequency spacing is reduced. Accordingly, the numberof subcarriers per unit frequency is increased, but the length of anOFDM symbol is increased.

That is, if a larger FFT size is used, it means that subcarrier spacingis narrowed. Likewise, it means that an inverse discrete Fouriertransform (IDFT)/discrete Fourier transform (DFT) period is increased.In this case, the IDFT/DFT period may mean a symbol length other than aguard interval (GI) in an OFDM symbol.

Accordingly, if an FFT size four times larger than that of the L-part isused in the HE-part (more specifically, the HE-preamble and the HE-datafield), the subcarrier spacing of the HE-part becomes ¼ times thesubcarrier spacing of the L-part, and the IDFT/DFT period of the HE-partis four times the IDFT/DFT period of the L-part. For example, if thesubcarrier spacing of the L-part is 312.5 kHz (=20 MHz/64, 40 MHz/128,80 MHz/256 and/or 160 MHz/512), the subcarrier spacing of the HE-partmay be 78.125 kHz (=20 MHz/256, 40 MHz/512, 80 MHz/1024 and/or 160MHz/2048). Furthermore, if the IDFT/DFT period of the L-part is 3.2 μs(=1/312.5 kHz), the IDFT/DFT period of the HE-part may be 12.8 μs(=1/78.125 kHz).

In this case, since one of 0.8 ρs, 1.6 and 3.2 μs may be used as a GI,the OFDM symbol length (or symbol interval) of the HE-part including theGI may be 13.6 μs, 14.4 μs, or 16 μs depending on the GI.

Referring to FIG. 16(b), the HE-SIG field may be divided into a HE-SIG-Afield and a HE-SIG-B field.

For example, the HE-part of the HE format PPDU may include a HE-SIG-Afield having a length of 12.8 μs, an HE-STF of 1 OFDM symbol, one ormore HE-LTFs, and a HE-SIG-B field of 1 OFDM symbol.

Furthermore, in the HE-part, an FFT size four times larger than that ofthe existing PPDU may be applied from the HE-STF other than the HE-SIG-Afield. That is, FFTs having 256, 512, 1024, and 2048 sizes may beapplied from the HE-STFs of the HE format PPDUs of 20 MHz, 40 MHz, 80MHz, and 160 MHz, respectively.

In this case, if the HE-SIG field is divided into the HE-SIG-A field andthe HE-SIG-B field as in FIG. 16(b), the positions of the HE-SIG-A fieldand the HE-SIG-B field may be different from those of FIG. 25(b). Forexample, the HE-SIG-B field may be transmitted after the HE-SIG-A field,and the HE-STF and the HE-LTF may be transmitted after the HE-SIG-Bfield. In this case, an FFT size four times larger than that of theexisting PPDU may be applied from the HE-STF.

Referring to FIG. 16(c), the HE-SIG field may not be divided into aHE-SIG-A field and a HE-SIG-B field.

For example, the HE-part of the HE format PPDU may include an HE-STF of1 OFDM symbol, a HE-SIG field of 1 OFDM symbol, and one or more HE-LTFs.

In the manner similar to that described above, an FFT size four timeslarger than that of the existing PPDU may be applied to the HE-part.That is, FFT sizes of 256, 512, 1024, and 2048 may be applied from theHE-STF of the HE format PPDU of 20 MHz, 40 MHz, 80 MHz, and 160 MHz,respectively.

Referring to FIG. 16(d), the HE-SIG field is not divided into a HE-SIG-Afield and a HE-SIG-B field, and the HE-LTF may be omitted.

For example, the HE-part of the HE format PPDU may include an HE-STF of1 OFDM symbol and a HE-SIG field of 1 OFDM symbol.

In the manner similar to that described above, an FFT size four timeslarger than that of the existing PPDU may be applied to the HE-part.That is, FFT sizes of 256, 512, 1024, and 2048 may be applied from theHE-STF of the HE format PPDU of 20 MHz, 40 MHz, 80 MHz, and 160 MHz,respectively.

The HE format PPDU for the WLAN system to which the present inventionmay be applied may be transmitted through at least one 20 MHz channel.For example, the HE format PPDU may be transmitted in the 40 MHz, 80 MHzor 160 MHz frequency band through total four 20 MHz channel. This willbe described in more detail with reference to the drawing below.

A HE-format PPDU for a WLAN system to which the present invention may beapplied may be transmitted over at least one 20 MHz channel. Forexample, a HE format PPDU may be transmitted in the 40 MHz, 80 MHz or160 MHz frequency band over a total of four 20 MHz channels. This willbe described in more detail with reference to the following drawings.

Hereinafter, the PPDU format will be described with reference to FIG. 25(b) for convenience of description, but the present invention is notlimited thereto.

FIG. 17 is a diagram illustrating a HE format PPDU according to anembodiment of the present invention.

FIG. 17 illustrates a PPDU format when 80 MHz is allocated to one STA(or OFDMA resource units are allocated to multiple STAs within 80 MHz)or when different streams of 80 MHz are allocated to multiple STAs,respectively.

Referring to FIG. 17, an L-STF, an L-LTF, and an L-SIG may betransmitted an OFDM symbol generated on the basis of 64 FFT points (or64 subcarriers) in each 20 MHz channel.

The HE-SIG B field may be positioned after the HE-SIG A field. In thiscase, an FFT size per unit frequency may be further increased after theHE-SFT (or HE-SIG B). For example, from the HE-STF (or HE-SIG-B), 256FFTs may be used in the 20 MHz channel, 512 FFTs may be used in the 40MHz channel, and 1024 FFTs may be used in the 80 MHz channel.

A HE-SIG-A field may include common control information commonlyreceived by STAs which receive a PPDU. The HE-SIG-A field may betransmitted in 1 to 3 OFDM symbols. The HE-SIG-A field is duplicated in20 MHz unit and contains the same information. The HE-SIG-A fieldindicates full bandwidth information of the system.

Table 7 illustrates information contained in the HE-SIG-A field.

TABLE 7 FIELD BITS DESCRIPTION Bandwidth 2 Indicates a bandwidth inwhich a PPDU is transmitted. For example, 20 MHz, 40 MHz, 80 MHz or 160MHz Group ID 6 Indicates as STA or a group of STAs that will receive aPPDU Stream 12 Indicates the number of location of spatial streams foreach STA or information the number of location of spatial streams for agroup of STAs UL indication 1 Indicates whether a PPDU is destined to anAP (uplink) or STA (downlink) MU indication 1 Indicates whether a PPDUis an SU-MIMO PPDU or an MU-MIMO PPDU GI indication 1 Indicates whethera short GI or a long GI is used Allocation 12 Indicates a band or achannel (subchannel index or subband index) information allocated toeach STA in a bandwidth in which a PPDU is transmitted Transmission 12Indicates a trasmission power for each channel or each STA power

Information contained in each of the fields illustrated in Table 7 maybe as defined in the IEEE 802.11 system. The above-described fields areexamples of the fields that may be included in the PPDU but not limitedto them. That is, the above-described fields may be substituted withother fields or further include additional fields, and not all of thefields may be necessarily included. Another example of informationincluded in the HE-SIG A field will be described hereinafter in relationto FIG. 34.

The HE-STF field is used to improve AGC estimation in MIMO transmission.

The HE-SIG-B field may include user-specific information that isrequired for each STA to receive its own data (i.e., a Physical LayerService Data Unit (PSDU)). The HE-SIG-B field may be transmitted in oneor two OFDM symbols. For example, the HE-SIG-B field may includeinformation about the length of a corresponding PSDU and the Modulationand Coding Scheme (MCS) of the corresponding PSDU.

The L-STF field, the L-LTF field, the L-SIG field, and the HE-SIG-Afield may be duplicately transmitted every 20 MHz channel. For example,when a PPDU is transmitted through four 20 MHz channels, the L-STFfield, the L-LTF field, L-STG field, and the HE-SIG-A field may beduplicately transmitted every 20 MHz channel.

If the FFT size is increased, a legacy STA that supports conventionalIEEE 802.11a/g/n/ac may be unable to decode a corresponding PPDU. Forcoexistence between a legacy STA and a HE STA, the L-STF, L-LTF, andL-SIG fields are transmitted through 64 FFT in a 20 MHz channel so thatthey can be received by a legacy STA. For example, the L-SIG field mayoccupy a single OFDM symbol, a single OFDM symbol time may be 4 μs, anda GI may be 0.8 μs.

An FFT size per unit frequency may be further increased from the HE-STF(or from the HE-SIG-A). For example, 256 FFT may be used in a 20 MHzchannel, 512 FFT may be used in a 40 MHz channel, and 1024 FFT may beused in an 80 MHz channel. If the FFT size is increased, the number ofOFDM subcarriers per unit frequency is increased because spacing betweenOFDM subcarriers is reduced, but an OFDM symbol time may be increased.In order to improve system efficiency, the length of a GI after theHE-STF may be set equal to the length of the GI of the HE-SIG-A.

The HE-SIG-A field includes information that is required for a HE STA todecode a HE PPDU. However, the HE-SIG-A field may be transmitted through64 FFT in a 20 MHz channel so that it may be received by both a legacySTA and a HE STA. The reason for this is that a HE STA is capable ofreceiving conventional HT/VHT format PPDUs in addition to a HE formatPPDU. In this case, it is required that a legacy STA and a HE STAdistinguish a HE format PPDU from an HT/VHT format PPDU, and vice versa.

FIG. 18 is a drawing illustrating an HE format PPDU according to anembodiment of the present invention.

In FIG. 18, it is assumed that 20 MHz channels are allocated todifferent STAs (e.g., STA 1, STA 2, STA 3, and STA 4).

Referring to FIG. 18, an FFT size per unit frequency may be furtherincreased from the HE-SFT (or the HE-SIG-B). For example, from theHE-STF (or HE-SIG-B), 256 FFTs may be used in the 20 MHz channel, 512FFTs may be used in the 40 MHz channel, and 1024 FFTs may be used in the80 MHz channel.

Information transmitted in each field included in a PPDU is the same asthe example of FIG. 26, and thus, descriptions thereof will be omittedhereinafter.

The HE-SIG-B may include information specified to each STA but it may beencoded in the entire band (i.e., indicated in the HE-SIG-A field). Thatis, the HE-SIG-B field includes information regarding every STA andevery STA receives the HE-SIG-B field.

The HE-SIG-B field may provide frequency bandwidth information allocatedto each STA and/or stream information in a corresponding frequency band.For example, in FIG. 27, as for the HE-SIG-B, STA 1 may be allocated 20MHz, STA 2 may be allocated a next 20 MHz, STA 3 may be allocated a next20 MHz, and STA 4 may be allocated a next 20 MHz. The STA 1 and STA 2may be allocated 40 MHz and STA 3 and STA 4 may be allocated a next 40MHz. In this case, STA 1 and STA 2 may be allocated different streamsand STA 3 and STA 4 may be allocated different streams.

Furthermore, an HE-SIG C field may be defined and added to the exampleof FIG. 27. In this case, information regarding every STA may betransmitted in the entire band in the HE-SIG-B field, and controlinformation specified to each STA may be transmitted by 20 MHz throughthe HE-SIG-C field.

Furthermore, unlike the examples of FIGS. 17 and 18, the HE-SIG-B fieldmay not be transmitted in the entire band but may be transmitted by 20MHz, like the HE-SIG-A field. This will be described with reference tothe following drawings.

FIG. 19 is a diagram illustrating an HE format PPDU according to anembodiment of the present invention.

In FIG. 19, it is assumed that 20 MHz channels are allocated todifferent STAs (e.g., STA 1, STA 2, STA 3, and STA 4).

Referring to FIG. 19, the HE-SIG-B field is not transmitted in theentire band but is transmitted by 20 MHz, like the HE-SIG-A field. Inthis case, however, unlike the HE-SIG-A field, the HE-SIG-B field may beencoded by 20 MHz and transmitted but may not be duplicated by 20 MHzand transmitted.

In this case, an FFT size per unit frequency may be further increasedfrom the HE-STF (or the HE-SIG-B). For example, from the HE-STF (orHE-SIG-B), 256 FFTs may be used in the 20 MHz channel, 512 FFTs may beused in the 40 MHz channel, and 1024 FFTs may be used in the 80 MHzchannel.

Information transmitted in each field included in the PPDU is the sameas the example of FIG. 18, and thus, descriptions thereof will beomitted.

The HE-SIG-A field is duplicated by 20 MHz and transmitted.

The HE-SIG-B field may provide frequency bandwidth information allocatedto each STA and/or stream information in a corresponding frequency band.Since the HE-SIG-B field includes information regarding each STA,information regarding each STA may be included in each HE-SIG-B field inunits of 20 MHz. In this case, in the example of FIG. 28, 20 MHz isallocated to each STA, but, in a case in which 40 MHz is allocated to anSTA, the HE-SIG-B may be duplicated by 20 MHz and transmitted.

In a case where a partial bandwidth having a low level of interferencefrom an adjacent BSS is allocated to an STA in a situation in which eachBSS supports different bandwidths, the HE-SIG-B is preferably nottransmitted in the entire band as mentioned above.

Hereinafter, the HE format PPDU of FIG. 28 will be described for thepurposes of description.

In FIGS. 17 to 19, a data field, as payload, may include a servicefield, a scrambled PSDU, a tail bit, and a padding bit.

Meanwhile, the HE format PPDU illustrated in FIGS. 17 to 19 may bedistinguished through a repeated L-SIG (RL-SIG), a repeated symbol of anL-SIG field. The RL-SIG field is inserted in front of the HE SIG-Afield, and each STA may identify a format of a received PPDU using theRL-SIG field, as an HE format PPDU.

A multi-user UL transmission method in a WLAN system is described below.

A method of transmitting, by an AP operating in a WLAN system, data to aplurality of STAs on the same time resource may be called downlinkmulti-user (DL MU) transmission. In contrast, a method of transmitting,by a plurality of STAs operating in a WLAN system, data to an AP on thesame time resource may be called uplink multi-user (UL MU) transmission.

Such DL MU transmission or UL MU transmission may be multiplexed on afrequency domain or a space domain.

If DL MU transmission or UL MU transmission is multiplexed on thefrequency domain, different frequency resources (e.g., subcarriers ortones) may be allocated to each of a plurality of STAs as DL or ULresources based on orthogonal frequency division multiplexing (OFDMA). Atransmission method through different frequency resources in such thesame time resources may be called “DL/UL MU OFDMA transmission.”

If DL MU transmission or UL MU transmission is multiplexed on the spacedomain, different spatial streams may be allocated to each of aplurality of STAs as DL or UL resources. A transmission method throughdifferent spatial streams on such the same time resources may be called“DL/UL MU MIMO transmission.”

Current WLAN systems do not support UL MU transmission due to thefollowing constraints.

Current WLAN systems do not support synchronization for the transmissiontiming of UL data transmitted by a plurality of STAs. For example,assuming that a plurality of STAs transmits UL data through the sametime resources in the existing WLAN system, in the present WLAN systems,each of a plurality of STAs is unaware of the transmission timing of ULdata of another STA. Accordingly, an AP may not receive UL data fromeach of a plurality of STAs on the same time resource.

Furthermore, in the present WLAN systems, overlap may occur betweenfrequency resources used by a plurality of STAs in order to transmit ULdata. For example, if a plurality of STAs has different oscillators,frequency offsets may be different. If a plurality of STAs havingdifferent frequency offsets performs UL transmission at the same timethrough different frequency resources, frequency regions used by aplurality of STAs may partially overlap.

Furthermore, in existing WLAN systems, power control is not performed oneach of a plurality of STAs. An AP dependent on the distance betweeneach of a plurality of STAs and the AP and a channel environment mayreceive signals of different power from a plurality of STAs. In thiscase, a signal having weak power may not be relatively detected by theAP compared to a signal having strong power.

Accordingly, an embodiment of the present invention proposes an UL MUtransmission method in a WLAN system.

FIG. 20 is a diagram illustrating an uplink multi-user transmissionprocedure according to an embodiment of the present invention.

Referring to FIG. 20, an AP may instruct STAs participating in UL MUtransmission to prepare for UL MU transmission, receive an UL MU dataframe from these STAs, and send an ACK frame (BA (Block Ack) frame) inresponse to the UL MU data frame.

First of all, the AP instructs STAs that will transmit UL MU data toprepare for UL MU transmission by sending an UL MU Trigger frame 2010.In this case, the term UL MU scheduling frame may be called “UL MUscheduling frame”.

In this case, the UL MU Trigger frame 2010 may contain controlinformation such as STA ID (identifier)/address information, informationon the allocation of resources to be used by each STA, and durationinformation.

The STA ID/address information refers to information on the identifieror address for specifying an STA that transmits uplink data.

The resource allocation information refers to information on uplinktransmission resources allocated to each STA (e.g., information onfrequency/subcarriers allocated to each STA in the case of UL MU OFDMAtransmission and a stream index allocated to each STA in the case of ULMU MIMO transmission).

The duration information refers to information for determining timeresources for transmitting an uplink data frame sent by each of multipleSTAs.

For example, the duration information may include period information ofa TXOP (Transmit Opportunity) allocated for uplink transmission of eachSTA or information (e.g., bits or symbols) on the uplink frame length.

The UL MU Trigger frame 2010 may further include control informationsuch as information on an MCS to be used when each STA sends an UL MUdata frame, coding information, etc.

The above-mentioned control information may be transmitted in a HE-part(e.g., the HE-SIG-A field or HE-SIG-B field) of a PPDU for deliveringthe UL MU Trigger frame 2010 or in the control field of the UL MUTrigger frame 2010 (e.g., the Frame Control field of the MAC frame).

The PPDU for delivering the UL MU Trigger frame 2010 starts with anL-part (e.g., the L-STF field, L-LTF field, and L-SIG field).Accordingly, legacy STAs may set their NAV (Network Allocation Vector)by L-SIG protection through the L-SIG field. For example, in the L-SIG,legacy STAs may calculate a period for NAV setting (hereinafter, ‘L-SIGprotection period’) based on the data length and data rate. The legacySTAs may determine that there is no data to be transmitted to themselvesduring the calculated L-SIG protection period.

For example, the L-SIG protection period may be determined as the sum ofthe value of the MAC Duration field of the UL MU Trigger frame 2010 andthe remaining portion after the L-SIG field of the PPDU delivering theUL MU Trigger frame 2010. Accordingly, the L-SIG protection period maybe set to a period of time until the transmission of an ACK frame 2030(or BA frame) transmitted to each STA, depending on the MAC durationvalue of the UL MU Trigger frame 2010.

Hereinafter, a method of resource allocation to each STA for UL MUtransmission will be described in more detail. A field containingcontrol information will be described separately for convenience ofexplanation, but the present invention is not limited to this.

A first field may indicate UL MU OFDMA transmission and UL MU MIMOtransmission in different ways. For example, ‘0’ may indicate UL MUOFDMA transmission, and ‘1’ may indicate UL MU MIMO transmission. Thefirst field may be 1 bit in size.

A second field (e.g., STA ID/address field) indicates the IDs oraddresses of STAs that will participate in UL MU transmission. The sizeof the second field may be obtained by multiplying the number of bitsfor indicating an STA ID by the number of STAs participating in UL MU.For example, if the second field has 12 bits, the ID/address of each STAmay be indicated in 4 bits.

A third field (e.g., resource allocation field) indicates a resourceregion allocated to each STA for UL MU transmission. Each STA may besequentially informed of the resource region allocated to it accordingto the order in the second field.

If the first field has a value of 0, this indicates frequencyinformation (e.g., frequency index, subcarrier index, etc.) for UL MUtransmission in the order of STA IDs/addresses in the second field, andif the first field has a value of 1, this indicates MIMO information(e.g., stream index, etc.) for UL MU transmission in the order of STAIDs/addresses in the second field.

In this case, a single STA may be informed of multiple indices (i.e.,frequency/subcarrier indices or stream indices). Thus, the third fieldmay be configured by multiplying the number of bits (or which may beconfigured in a bitmap format) by the number of STAs participating in ULMU transmission.

For example, it is assumed that the second field is set in the order ofSTA 1, STA 2, . . . , and the third field is set in the order of 2, 2, .. . .

In this case, if the first field is 0, frequency resources may beallocated to STA 1 and STA 2, sequentially in the order of higherfrequency region (or lower frequency region). In an example, when 20 MHzOFDMA is supported in an 80 MHz band, STA 1 may use a higher (or lower)40 MHz band and STA 2 may use the subsequent 40 MHz band.

On the other hand, if the first field is 1, streams may be allocated toSTA 1 and STA 2, sequentially in the order of higher-order (orlower-order) streams. In this case, a beamforming scheme for each streammay be prescribed, or the third field or fourth field may contain morespecific information on the beamforming scheme for each stream.

Each STA sends a UL MU Data frame 2021, 2022, and 2023 to an AP based onthe UL MU Trigger frame 2010. That is, each STA may send a UL MU Dataframe 2021, 2022, and 2023 to an AP after receiving the UL MU Triggerframe 2010 from the AP.

Each STA may determine particular frequency resources for UL MU OFDMAtransmission or spatial streams for UL MU MIMO transmission, based onthe resource allocation information in the UL MU Trigger frame 2010.

Specifically, for UL MU OFDMA transmission, each STA may send an uplinkdata frame on the same time resource through a different frequencyresource.

In this case, each of STA 1 to STA 3 may be allocated differentfrequency resources for uplink data frame transmission, based on the STAID/address information and resource allocation information included inthe UL MU Trigger frame 2010. For example, the STA ID/addressinformation may sequentially indicate STA 1 to STA 3, and the resourceallocation information may sequentially indicate frequency resource 1,frequency resource 2, and frequency resource 3. In this case, STA 1 toSTA 3 sequentially indicated based on the STA ID/address information maybe allocated frequency resource 1, frequency resource 2, and frequencyresource 3, which are sequentially indicated based on the resourceallocation information. That is, STA 1, STA 2, and STA 3 may send theuplink data frame 2021, 2022, and 2023 to the AP through frequencyresource 1, frequency resource 2, and frequency resource 3,respectively.

For UL MU MIMO transmission, each STA may send an uplink data frame onthe same time resource through at least one different stream among aplurality of spatial streams.

In this case, each of STA 1 to STA 3 may be allocated spatial streamsfor uplink data frame transmission, based on the STA ID/addressinformation and resource allocation information included in the UL MUTrigger frame 2010. For example, the STA ID/address information maysequentially indicate STA 1 to STA 3, and the resource allocationinformation may sequentially indicate spatial stream 1, spatial stream2, and spatial stream 3. In this case, STA 1 to STA 3 sequentiallyindicated based on the STA ID/address information may be allocatedspatial stream 1, spatial stream 2, and spatial stream 3, which aresequentially indicated based on the resource allocation information.That is, STA 1, STA 2, and STA 3 may send the uplink data frame 2021,2022, and 2023 to the AP through spatial stream 1, spatial stream 2, andspatial stream 3, respectively.

The PPDU for delivering the uplink data frame 2021, 2022, and 2023 mayhave a new structure, even without an L-part.

For UL MU MIMO transmission or for UL MU OFDMA transmission in a subbandbelow 20 MHz, the L-part of the PPDU for delivering the uplink dataframe 2021, 2022, and 2023 may be transmitted on an SFN (that is, allSTAs send an L-part having the same configuration and content). On thecontrary, for UL MU OFDMA transmission in a subband above 20 MHz, theL-part of the PPDU for delivering the uplink data frame 2021, 2022, and2023 may be transmitted every 20 MHz.

As long as the information in the UL MU Trigger frame 2010 suffices toconstruct an uplink data frame, the HE-SIG field (i.e., a part wherecontrol information for a data frame configuration scheme istransmitted) in the PPDU delivering the uplink data frame 2021, 2022,and 2023 may not be required. For example, the HE-SIG-A field and/or theHE-SIG-B field may not be transmitted. The HE-SIG-A field and the HE-SIGC field may be transmitted, but the HE-SIG-B field may not betransmitted.

An AP may send an ACK Frame 2030 (or BA frame) in response to the uplinkdata frame 2021, 2022, and 2023 received from each STA. In this case,the AP may receive the uplink data frame 2021, 2022, and 2023 from eachSTA and then, after an SIFS, transmit the ACK frame 2030 to each STA.

Using the existing ACK frame structure, an RA field having a size of 6octets may include the AID (or Partial AID) of STAs participating in ULMU transmission.

Alternatively, an ACK frame with a new structure may be configured forDL SU transmission or DL MU transmission.

The AP may send an ACK frame 2030 to an STA only when an UL MU dataframe is successfully received by the corresponding STA. Through the ACKframe 2030, the AP may inform whether the reception is successful or notby ACK or NACK. If the ACK frame 2030 contains NACK information, it alsomay include the reason for NACK or information (e.g., UL MU schedulinginformation, etc.) for the subsequent procedure.

Alternatively, the PPDU for delivering the ACK frame 2030 may beconfigured to have a new structure without an L-part.

The ACK frame 2030 may contain STA ID or address information, but theSTA ID or address information may be omitted if the order of STAsindicated in the UL MU Trigger frame 2010 also applies to the ACK frame2030.

Moreover, the TXOP (i.e., L-SIG protection period) of the ACK frame 2030may be extended, and a frame for the next UL MU scheduling or a controlframe containing adjustment information for the next UL MU transmissionmay be included in the TXOP.

Meanwhile, an adjustment process may be added to synchronize STAs for ULMU transmission.

So far, the IEEE 802.11ax WLAN system has been described. Hereinafter, aDL/UL MU data transmission method according to an embodiment of thepresent invention will be described.

FIG. 21 illustrates UL MU transmission according to an embodiment of thepresent invention.

Referring to FIG. 21(a), when an AP sends a trigger frame, each of STAsmay send UL MU data. However, some STAs within a BSS may not recognizethe presence of an UL MU frame.

More specifically, other STA 1 receives the trigger frame within theBSS, but may not receive the UL MU frame. Accordingly, after a lapse ofan EIFS (EIFS=aSIFSTime+DIFS+EstimatedACKTxTime) after the trigger frameis received, other STA 1 may send UL data to the AP. In the case of alegacy 801.11 system, the AP may complete the transmission of an ACKframe within the EIFS after receiving an UL MU frame. However, the ULframe transmitted by other STA 1 after the EIFS may collide against ULMU data communication according to the trigger frame because an UL MUpacket in an 11ax system may have a longer length than the legacysystem.

Referring to FIG. 21(b), an AP sends a trigger frame and each of STAssends UL MU data. However, some STAs within an OBSS may not recognizethe presence of the trigger frame and an ACK frame.

More specifically, other STA 2 does not overhear the trigger frame, butmay overhear only the UL MU frame. Accordingly, other STA 2 may send itsown packet after a lapse of the EIFS from the end of the UL MU frame.However, the frame transmitted by other STA 2 may collide against theACK frames transmitted by MU STAs because the length of the DL MU ACKframe is longer than the length of a legacy ACK/BA frame.

In an UL MU procedure, such as that shown in FIG. 21, there is a needfor additional TXOP protection for preventing a collision with thetransmission data of another STA. A TXOP protection method proposed bythe present invention for such an UL MU procedure is described below.

FIG. 22 is a diagram illustrating a CTS-to-self frame according to anembodiment of the present invention.

If a trigger frame is transmitted in the MAC frame format of a legacysystem, a legacy STA may read the duration field of a MAC header withinthe trigger frame and perform NAV setting. However, if a trigger frameis transmitted in the MAC frame format of an 11ax system, a legacy STAis unable to perform NAV setting because it can read only fields up toan L-SIG field.

Accordingly, an embodiment of the present invention proposes that an APforces the transmission of CTS-to-self prior to the transmission of atrigger frame if the trigger frame is configured with the MAC frameformat of an 11ax system. In this case, “CTS-to-self” indicates a frameindicating that surrounding STAs can set a TXOP interval by allowingeach STA to insert its own address into the RA field of a CTS frame andto send the CTS frame. A legacy STA may receive CTS-to-self and performNAV setting for an UL MU procedure (or may set a TXOP interval).

Furthermore, upon CTS-to-self transmission, the concept of bandwidthsignaling TA applied to only the TA field of a legacy system may beintroduced. In this case, “bandwidth signaling TA” is a methodindicating that a corresponding frame includes bandwidth (BW)information by setting the MSB (1 bit) (this is basically a bitproviding notification of an individual/group) of a TA field in an RTS,ACK, BAR, BA, NDPA, a Poll or a BF-poll frame, to ‘1’. In a legacysystem, although the TA field of an RTS frame has been set to ‘1’, theTA field of a CTS frame for the corresponding RTS frame is set to ‘0’and transmitted.

Likewise, in an embodiment of the present invention, the MSB (1 bit) ofan RA field included in CTS-to-self may be set to ‘1’ in order toprovide notification that the CTS-to-self includes the entire bandwidthof a trigger frame or an UL MU procedure (refer to FIG. 22). In thiscase, however, this may be interpreted as a method of using the MSB (1bit) of an RA field in a legacy system (i.e., it may be determined to bebroadcast information other than BW signaling information). Accordingly,only when an STA receives a trigger frame after CTS-to-self, it maydetermine that the MSB of the RA field within the CTS-to-self indicatesthat “the CTS-to-self includes bandwidth information.” In this case, theSTA may obtain information about a full band of the trigger frame or anUL MU procedure through the CTS-to-self.

Alternatively, since the RA field of CTS-to-self includes a BSSID, an APmay set the MSB (1 bit) of the RA field of a CTS frame to ‘1’ and mayset the remaining LSBs as a BSSID (or at least part of the BSSID). Inthis case, an STA may recognize that the corresponding CTS-to-self is aCTS-to-self frame for bandwidth signaling.

The format of an UL MU PPDU (or UL MU frame) transmitted through atransmission channel having a size exceeding 20 MHz is proposed.

Structure of UL MU PPDU in 802.11ax System

FIG. 23 shows the structure of the UL MU PPDU of an HE format accordingto an embodiment of the present invention.

Referring to FIG. 23, the UL MU PPDU (or UL MU frame) may be basicallydivided into a first part (or an area A) and a second part (or an areaB). In this case, the first part and the second part may be classifiedbased on an IDFT/DFT period. For example, the first part may be aportion having a first IDFT/DFT period (e.g., 3.2 μs), and the secondpart may be a portion having a second IDFT/DFT period (e.g., 12.8 μs)that is four times the first IDFT/DFT period. Accordingly, the firstpart may be a portion including an L-STF, an L-LTF and/or an L-SIGfield, and the second part may be a portion including an HE-STF, anHE-LTF, an HE-SIG-C field and/or a data field (or A-MPDU), but thepresent invention is not limited thereto. A new field (e.g., an HE-SIG Bfield or an RL-SIG field) may be added to each part or a specific fieldmay be omitted from each field.

As described above in relation to FIG. 19, the second part may besubjected to UL MU transmission using frequency/space resourcesallocated to each STA. In the current rule, the first part may beduplicated in 20 MHz unit and transmitted, but a detailed rule regardingthat how the first part will be duplicated and transmitted has not beendetermined so far. Accordingly, hereinafter, there is proposed anefficient UL MU transmission method of the first part.

For convenience of description, it is hereinafter assumed that a DL MUPPDU on which a trigger frame including resource allocation informationabout a plurality of STAs is carried has been transmitted to each STAthrough an 80 MHz channel.

1. A First Embodiment—Duplication of Each 20 MHz Over a Full Band

FIG. 24 is a diagram showing the structure of an UL MU PPDU according toa first embodiment of the present invention.

Referring to FIG. 24, the first part of the UL MU PPDU may be duplicatedin 20 MHz unit and transmitted over a full band. In other words, thefirst part of the UL MU PPDU may be duplicated in 20 MHz unit andtransmitted over full transmission channel of the UL MU PPDU.

For example, it may be assumed that when the DL MU PPDU (including atrigger frame) is received through the 80 MHz channel, a fulltransmission band of the UL MU PPDU has also been determined to be thesame 80 MHz channel. In this case, the first part may be duplicated fourtimes in 20 MHz unit and may be subjected to UL MU transmission throughthe 80 MHz channel (or using the 80 MHz channel). If the DL MU PPDU isreceived through a 40 MHz channel, the first part may be duplicatedtwice in 20 MHz unit and may be subjected to UL MU transmission throughthe 40 MHz channel.

In the case of the first embodiment, although some of STAs that send ULMU PPDUs fail in UL MU transmission, a power imbalance problem is notgenerated for each band (e.g., per 20 MHz channel) because the firstpart is transmitted over a full band (or all of transmission channels)by another STA. Furthermore, since the first part is transmitted overthe full band, an empty band (e.g., an empty 20 MHz channel) is notpresent. Accordingly, there is an advantage in that a full band (all oftransmission channels) can be subjected to TXOP protection if the TXOPprotection is performed using an L-SIG or HE-SIG A field.

The second part may be transmitted using a frequency resource allocatedto each STA based on frequency resource allocation information of atrigger frame received through the DL MU PPDU.

2. A Second Embodiment—Transmission through a Primary Channel

FIG. 25 is a diagram showing the structure of an UL MU PPDU according toa second embodiment of the present invention.

Referring to FIG. 25, the first part of the UL MU PPDU may betransmitted through a primary channel. This may be classified into threedifferent embodiments. In this case, the primary channel indicates achannel (e.g., a primary 20 MHz, 40 MHz or 80 MHz channel) through whichCCA and/or a backoff count is performed in order for a legacy STA todetermine whether it may send data or not. In the case of the remainingchannels (e.g., a secondary 20 MHz, 40 MHz or 80 MHz channel), thelegacy STA checks whether the remaining channels are idle prior to aPIFS and sends data.

In one embodiment, the first part of the UL MU PPDU may be transmittedthrough only the primary channel. For example, it may be assumed thatwhen the DL MU PPDU (including a trigger frame) is received through the80 MHz channel, a full transmission band of the UL MU PPDU has also beendetermined to be the same 80 MHz channel. In this case, the first partmay be subjected to UL MU transmission through the primary channel(e.g., the primary 20 MHz channel) within the corresponding 80 MHzchannel.

In another embodiment, the first part of the UL MU PPDU may betransmitted through the primary channel and the channel(s) of a locationcorresponding to a frequency resource (or a frequency resource for thetransmission of the second part) allocated to an STA through the triggerframe.

For example, it may be assumed that when the DL MU PPDU (including atrigger frame) is received through the 80 MHz channel, a fulltransmission band of the UL MU PPDU has also been determined to be thesame 80 MHz channel. In this case, the first part may be transmittedthrough the primary channel within the corresponding 80 MHz channel andthe 20 MHz channel(s) (i.e., the 20 MHz channel of a locationcorresponding to a frequency resource allocated for the transmission ofthe second part or a channel including at least some of a frequencyresource indicated by resource allocation information, hereinafterreferred to as a “corresponding channel”) of a location corresponding toa frequency resource indicated by the trigger frame. In the presentembodiment, the first part may be subjected to UL MU transmissionthrough a channel(s) discontinuously located in a frequency area (if theprimary channel and the corresponding channel(s) are discontinuouslylocated in the frequency area).

In another embodiment, the first part may be subjected to UL MUtransmission through channels continuously located in a frequency areafrom the primary channel to the corresponding channels. In other words,in yet another embodiment, the first part may be subjected to UL MUtransmission through a primary channel, a corresponding channel and atleast one 20 MHz channel located between the primary channel and thecorresponding channel. In the present embodiment, the first part may besubjected to UL MU transmission through channels continuously located ina frequency area (although a primary channel and the correspondingchannels are discontinuously located in the frequency area).

In the aforementioned examples, if the size of a channel through whichthe first part is to be transmitted exceeds 20 MHz (e.g., 40 MHz or 80MHz), the first part may be duplicated in 20 MHz unit by the size of thechannel and may be subjected to UL MU transmission through atransmission channel. For example, if the size of a channel throughwhich the first part is to be transmitted is 80 MHz, the first part maybe duplicated four times in 20 MHz unit and may be subjected to UL MUtransmission through a transmission channel, but the present inventionis not limited thereto. In some embodiments, the first part may beduplicated in 40 MHz or 80 MHz unit by the size of the channel and maybe subjected to UL MU transmission through a transmission channel.

In contrast, if the size of a channel through which the first part is tobe transmitted is 20 MHz or less, the first part is not separatelyduplicated and may be subjected to UL MU transmission through atransmission channel.

The second part may be transmitted using a frequency resource allocatedto each STA based on frequency resource allocation information of atrigger frame received through a DL MU PPDU.

If a primary channel within the transmission channel of an UL MU PPDU isempty, another STA may determine that the corresponding transmissionchannel is idle and attempt data transmission. Accordingly, if some STAsfail in the UL MU transmission of the first part through a primarychannel, another STA may attempt data transmission. In order to preventsuch a situation, according to the second embodiment, all of STAs sendthe first parts through the primary channel.

3. A Third Embodiment—Transmission through a Corresponding Channel

FIG. 26 is a diagram showing the structure of an UL MU PPDU according toa third embodiment of the present invention.

Referring to FIG. 26, the first part of the UL MU PPDU may betransmitted through a corresponding channel. In this case, as describedabove in relation to the second embodiment, the corresponding channelmay indicate the 20 MHz channel of a location corresponding to afrequency resource for the transmission of the second part, the 20 MHzchannel of a location corresponding to a frequency resource allocated toan STA through a trigger frame, the 20 MHz channel of a locationcorresponding to a frequency resource indicated by the resourceallocation information of a trigger frame or a 20 MHz channel includingat least some of a frequency resource indicated by resource allocationinformation.

Accordingly, in the present embodiment, the first part is notessentially subjected to UL MU transmission through a primary channel.

As shown in FIG. 26(a), the number of corresponding channels may be 1.As shown in FIG. 26(b), the number of corresponding channels may beplural. If the number of corresponding channels is plural, the firstpart may be duplicated in 20 MHz unit by the size of the correspondingchannel in 20 MHz unit and may be subjected to UL MU transmissionthrough a transmission channel. In contrast, if the size of a channelthrough which the first part is to be transmitted is 20 MHz or less, thefirst part is not separately duplicated and may be subjected to UL MUtransmission through a corresponding channel.

The second part may be transmitted using a frequency resource allocatedto each STA based on the frequency resource allocation information of atrigger frame received through a DL MU PPDU.

The present embodiment has an advantage in that it proposes an UL MUPPDU having small overhead and a simpler structure.

FIG. 27 is a flowchart regarding the DL MU transmission method of an APdevice according to an embodiment of the present invention. Thedescriptions regarding the aforementioned embodiments may be identicallyapplied to the flowchart of FIG. 27. Accordingly, a redundantdescription thereof is omitted.

First, the AP may generate a DL MU PPDU (S2710). In this case, thegenerated DL MU PPDU may correspond to a DL MU PPDU on which a triggerframe including resource allocation information for the UL MUtransmission of STAs is carried.

The AP may send the generated DL MU PPDU to the STAs (S2720).

The AP may receive an UL MU PPDU, generated based on the transmitted DLMU PPDU, from each of the STAs (S2730). In this case, the UL MU PPDUreceived from each STA may include the first and the second partclassified based on the IDFT/DFT period as described above in relationto FIG. 23.

The first part may have a first IDFT/DFT period (3.2 μs) and the secondpart may have a second IDFT/DFT period (12.8 μs), that is, four timesthe first IDFT/DFT period. The second part may be received using afrequency resource indicated by resource allocation information includedin a trigger frame. In various embodiments, the first part may besubjected to UL MU reception. For example, the first part may be i)duplicated in 20 MHz unit over a full band and subjected to UL MUreception, ii) subjected to UL MU reception through a primary channel oriii) UL MU reception through a corresponding channel. A detaileddescription related to the embodiments is the same as that given inrelation to FIGS. 24 to 26, and thus a redundant description thereof isomitted.

Furthermore, the aforementioned flowchart may also be similarly appliedto a case where the subject is an STA device.

First, the STA may receive a DL MU PPDU from an AP. In this case, thegenerated DL MU PPDU may correspond to a DL MU PPDU on which a triggerframe including resource allocation information for the UL MUtransmission of STAs is carried.

Next, the STA may generate and send an UL MU PPDU based on the receivedDL MU PPDU. In this case, a description of the generated UL MU PPDU isthe same as that given in relation to FIGS. 24 to 26.

FIG. 28 is a block diagram of each STA device according to an embodimentof the present invention.

In FIG. 28, an STA device 2810 may include a memory 2812, a processor2811 and an RF unit 2813. And, as described above, the STA device may bean AP or a non-AP STA as an HE STA device.

The RF unit 2813 may transmit/receive a radio signal with beingconnected to the processor 2811. The RF unit 2813 may transmit a signalby up-converting the data received from the processor 2811 to thetransmission/reception band.

The processor 2811 may implement the physical layer and/or the MAC layeraccording to the IEEE 802.11 system with being connected to the RF unit4013. The processor 2811 may be constructed to perform the operationaccording to the various embodiments of the present invention accordingto the drawings and description. In addition, the module forimplementing the operation of the STA 2810 according to the variousembodiments of the present invention described above may be stored inthe memory 2812 and executed by the processor 2811.

The memory 2812 is connected to the processor 2811, and stores varioustypes of information for executing the processor 2811. The memory 2812may be included interior of the processor 2811 or installed exterior ofthe processor 2811, and may be connected with the processor 2811 by awell known means.

In addition, the STA device 2810 may include a single antenna or amultiple antenna.

The detailed construction of the STA device 2810 of FIG. 28 may beimplemented such that the description of the various embodiments of thepresent invention is independently applied or two or more embodimentsare simultaneously applied.

The embodiments described above are constructed by combining elementsand features of the present invention in a predetermined form. Theelements or features may be considered optional unless explicitlymentioned otherwise. Each of the elements or features can be implementedwithout being combined with other elements. In addition, some elementsand/or features may be combined to configure an embodiment of thepresent invention. The sequential order of the operations discussed inthe embodiments of the present invention may be changed. Some elementsor features of one embodiment may also be included in anotherembodiment, or may be replaced by corresponding elements or features ofanother embodiment. Also, it will be obvious to those skilled in the artthat claims that are not explicitly cited in the appended claims may bepresented in combination as an exemplary embodiment of the presentinvention or included as a new claim by subsequent amendment after theapplication is filed.

The embodiments of the present invention may be implemented throughvarious means, for example, hardware, firmware, software, or acombination thereof. When implemented as hardware, one embodiment of thepresent invention may be carried out as one or more application specificintegrated circuits (ASICs), one or more digital signal processors(DSPs), one or more digital signal processing devices (DSPDs), one ormore programmable logic devices (PLDs), one or more field programmablegate arrays (FPGAs), a processor, a controller, a microcontroller, amicroprocessor, etc.

When implemented as firmware or software, one embodiment of the presentinvention may be carried out as a module, a procedure, or a functionthat performs the functions or operations described above. Software codemay be stored in the memory and executed by the processor. The memory islocated inside or outside the processor and may transmit and receivedata to and from the processor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above exemplary embodiments are therefore to beconstrued in all aspects as illustrative and not restrictive. The scopeof the invention should be determined by the appended claims and theirlegal equivalents, not by the above description, and all changes comingwithin the meaning and equivalency range of the appended claims areintended to be embraced therein

MODE FOR INVENTION

Various embodiments for carrying out the invention have been describedin the best mode for invention.

INDUSTRIAL APPLICABILITY

While a frame transmission scheme in a wireless communication systemaccording to the present invention has been described with respect toits application to an IEEE 802.11 system, it also may be applied toother various wireless communication systems than the IEEE 802.11system.

What is claimed is:
 1. An uplink (UL) multi-user (MU) transmission method of an STA in a wireless communication system, the method comprising: receiving a DL physical protocol data unit (PPDU), the DL PPDU comprising resource allocation information for an UL MU transmission of a station (STA); and sending an UL MU PPDU generated based on the DL PPDU, wherein the UL MU PPDU comprises a first part having a first inverse discrete Fourier transform (IDFT)/discrete Fourier transform (DFT) period and a second part having a second IDFT/DFT period which is four times the first IDFT/DFT period, wherein the second part is transmitted using a frequency resource indicated by the resource allocation information, wherein the first part is transmitted through at least one 20 MHz channel of a location related to the frequency resource indicated by the resource allocation information, and the first part is transmitted only through the at least one 20 MHz channel that includes a location where the second part is transmitted.
 2. The UL MU transmission method of claim 1, wherein: the first part comprises a legacy (L)-short training field (STF), an L-long training field (LTF), an L-signal (SIG) field, and a high efficiency (HE) SIG-A field, and the second part comprises an HE-STF, an HE-LTF, and a data field.
 3. A station (STA) device of a wireless LAN (WLAN) system, comprising: an RF unit configured to send or receive a radio signal; and a processor configured to control the RF unit, wherein the processor is further configured to receive a DL physical protocol data unit (PPDU), the DL PPDU comprising resource allocation information for an uplink (UL) multi-user (MU) transmission of the STA, and send an UL MU PPDU generated based on the DL PPDU, wherein the UL MU PPDU comprises a first part having a first inverse discrete Fourier transform (IDFT)/discrete Fourier transform (DFT) period and a second part having a second IDFT/DFT period which is four times the first IDFT/DFT period, wherein the second part is received using a frequency resource indicated by the resource allocation information wherein the first part is transmitted through at least one 20 MHz channel of a location related to the frequency resource indicated by the resource allocation information, and the first part is transmitted only through the at least one 20 MHz channel that includes a location where the second part is transmitted.
 4. The STA device of claim 3, wherein: the first part comprises a legacy (L)-short training field (STF), an L-long training field (LTF), an L-signal (SIG) field, and a high efficiency (HE) SIG-A field, the second part comprises an HE-STF, an HE-LTF, and a data field, and the resource allocation information is received in a trigger frame in the DL PPDU.
 5. The UL MU transmission method of claim 1, wherein the at least one 20 MHz channel is a primary channel or a secondary channel.
 6. The UL MU transmission method of claim 1, wherein, when the plurality of 20 MHz channels consists of one 20 MHz channel, the first part is transmitted through the one 20 MHz channel.
 7. The UL MU transmission method of claim 1, wherein, when the plurality of 20 MHz channels comprises two 20 MHz channels, the first part is duplicated in units of 20 MHz and is transmitted through the two of 20 MHz channels.
 8. The UL MU transmission method of claim 7, wherein, when the plurality of 20 MHz channels comprises two 20 MHz channels, the frequency resource indicated for the transmission of the second part: is larger than 20 MHz, or is less than the 20 MHz channel and is located at a boundary between the two 20 MHz channels used to transmit the first part.
 9. The UL MU transmission method of claim 2, wherein the resource allocation information is received in a trigger frame in the DL PPDU.
 10. The STA device of claim 3, wherein the at least one 20 MHz channel is a primary channel or a secondary channel.
 11. The STA device of claim 3, wherein, when the plurality of 20 MHz channels consists of one 20 MHz channel, the first part is transmitted through the one 20 MHz channel.
 12. The STA device of claim 3, wherein, when the plurality of 20 MHz channels comprises two 20 MHz channels, the first part is duplicated in units of 20 MHz and is transmitted through the two of 20 MHz channels.
 13. The STA device of claim 12, wherein, when the plurality of 20 MHz channels comprises two 20 MHz channels, the frequency resource indicated for the transmission of the second part: is larger than the 20 MHz, or is less than the 20 MHz channel and is located at a boundary between the two 20 MHz channels used to transmit the first part. 